Short answer

No. Adding unblessed water to blessed water does not ordinarily destroy or “cancel” the blessing. The resulting mixture may still properly be called blessed water, although it is more diluted. The blessing is not a chemical property residing in a particular molecule of water; it is a liturgical blessing of the water as a quantity or vessel.

The provided sources do not give a current universal Latin canon specifically governing ordinary household holy water. They do, however, support this conclusion by analogy and by the Church’s baptismal discipline.

Why the blessing is not simply lost

Holy water is a sacramental, not a sacrament. It is water blessed by the Church through prayer, so that those who use it may receive spiritual benefits according to their faith and disposition. The Baltimore Catechism defines it as:

Thus, its sacramental character comes from the Church’s prayer and blessing, not from a measurable physical ingredient. The Roman Missal’s blessing likewise asks God to bless the water so that it may serve as a memorial of Baptism and an occasion of grace. 2

Adding ordinary water therefore does not constitute an act of desecration or automatically invalidate the blessing. It simply increases the quantity of water that is associated with the original blessed water.

The strongest evidence: baptismal water

A historical canon from the 1917 Code explicitly addressed the replenishment of baptismal water:

That canon is not the current law for present-day Latin canon-law questions, since the 1983 Code is now in force. Nevertheless, it illustrates a longstanding Catholic principle: adding unblessed water to blessed water does not necessarily remove the sacramental blessing, especially when the blessed water remains the principal component.

The distinction is important:

• A small amount of ordinary water added to a larger amount of holy water: the mixture may reasonably continue to be treated as holy water.
• A large amount of ordinary water added to a very small amount of holy water: the resulting water may be so diluted that it would be prudent not to presume that the original blessing extends to the whole mixture.
• Water that has become contaminated, spoiled, or unsuitable for use: it should not be used merely because some blessed water was once added to it.

Ordinary holy water and baptismal water are not identical

The sources distinguish ordinary holy water from water prepared for Baptism.

Ordinary holy water is a sacramental used for such purposes as blessing persons, places, and religious objects. Baptismal water, by contrast, is prepared and blessed specifically for the sacrament. The Church’s baptismal instructions state that, except in necessity, the minister should use water blessed for the rite. 4

Therefore, one should not assume that a mixture of ordinary holy water and tap water is automatically suitable for the solemn celebration of Baptism. Baptism should follow the approved liturgical rite and use water blessed for that purpose, except in a genuine emergency. The Roman Missal describes the baptismal blessing as invoking the Holy Spirit upon the font so that the water may signify and serve the sacramental mystery of Baptism. 5

This does not mean that ordinary holy water becomes worthless when diluted. It means that the requirements for baptismal water are more specific than those for devotional holy water.

Practical guidance

For home use, if you accidentally or intentionally add clean, ordinary water to holy water:

1. Do not regard the holy water as spoiled or invalidated.
2. If the holy water was only slightly diluted, it is reasonable to continue using the mixture as holy water.
3. If it was greatly diluted, or if you are uncertain, use it devotionally but obtain fresh holy water from a parish for clearer assurance.
4. Do not use a questionable mixture for a baptism or other liturgical rite requiring properly blessed water.
5. Keep holy water in a clean, suitable container; if it becomes dirty or contaminated, dispose of it reverently rather than using it.

The Baltimore Catechism also notes that different kinds of water may be blessed with different degrees of solemnity—for example, Easter water is blessed with special rites—but this concerns the form and purpose of the blessing, not a claim that adding water causes the blessing to vanish instantly. 6

Conclusion

Adding unblessed water to blessed water does not ordinarily remove its sacramental status. The mixture remains suitable for ordinary devotional use when the blessed water remains a meaningful part of it and the water is clean. For Baptism or an official liturgical rite, however, use water blessed according to the prescribed rite rather than relying on a diluted household mixture.