Synthesis of superabsorbent hydrogel (SAHG) of acrylic acid-potassium acrylate copolymer by direct Ultraviolet (UV) photopolymerization. is a new method. The effects of degree of neutralization of acrylic acid (AA), photoinitiators, crosslinking agents, and exposure time of UV light on water absorbent properties were investigated. The results showed that the water absorbency (Q) and the salt solution (NaCl, 0.9%) absorbency (Qs) of SAHG, based on Irgacure 651, are high, reaching about 1400 and 130 mL/g, respectively. UV absorption spectrum proved that peak of UV absorption of Irgacure 651 matched the UV light source we used. Among the crosslinking agents, N,N'-methylene bisacrylamide is more efficient than others, because of its very small content and high Q. C-13 NMR spectrometry was used to identify the mechanism of crosslinking reaction through esterification of hydroxyethyl acrylate (HEA) and 2-hydroxypropyl acrylate (HPA) with carboxylic acid group in acrylic acid-ammonium acrylate copolymerization, but efficiency of crosslinking reaction by esterification was lower than that of copolymerization of vinyl group in crosslinking agent. The Q of acrylic acid-potassium acrylate copolymer of SAHG reaches 1592 mL/g under the following conditions: degree of neutralization of acrylic acid is 80%, content of Irgacure 651 is 0.25 wt %, content of HEA is 0.2 wt %, and exposure time is 10 min. (c) 2006 Wiley Periodicals, Inc.