PURPOSE: To provide a method for reducing the level of a carbonyl compd. in an aq. monomer soln. contg. a carbonyl compd. as an impurity.

CONSTITUTION: An aq. monomer soln. contg. an impurity is subjected to ultraviolet irradiation for about 2 min to about 5 hr. The aq. monomer soln. is pref. an aq. soln. of acrylic acid or methacrylic acid. The monomer concn. is most pref. about 20 wt.% to about 65 wt.%. Upon exposure of the monomer soln. to ultraviolet light, the fear of an desired polymer being produced increased with increasing the monomer concn. Carbonyl compds. present in the aq. monomer soln. include, e.g. acrolein, methacrolein, acetaldehyde, furfural, protoanemonin, formaldehyde, crotonaldehyde, acetone, and benz aldehyde.