A method for producing aqueous ortho-phthalaldehyde solutions which comprises introducing an acetal of the o-phthalaldehyde into water and subsequently setting a pH <7 by adding an acid, whereupon, then at a temperature between 10.degree. C. and 90.degree. C., the acetal is converted into the o-phthalaldehyde by eliminating the corresponding alcohol, and also use of OPA acetals for producing these solutions.