Environmentally friendly water-soluble photoinitiators are strongly desirable for the practical applications of photopolymerization. This article reports that aliphatic ketones and formaldehyde can be used as effective water-soluble photoinitiators for the photopolymerization of methacrylic acid (MAA) when they are mixed with an appropriate amount of water (and ethanol as a co-solvent in some cases). Among the aliphatic ketones, including acetone, butanone and pentan-2-one, butanone is the most effective. The aliphatic ketones usually exhibit the highest photoinitiation efficiency at a concentration of 5-10 vol.% and a high volume ratio of water to ethanol. By using acetone as the photoinitiator, the factors affecting the photopolymerization of MAA, including atmosphere, UV intensity and monomer concentration, are studied. Oxygen strongly inhibits the photopolymerization. The percent conversion and polymerization rate increase with the increase of UV intensity. The percent conversion decreases whereas polymerization rate increases with the increase of initial monomer concentration. (C) 2013 Elsevier Ltd. All rights reserved.