Hydrogels with semi-interpenetrating networks composed of poly(acrylic acid) (PAAc) and poly(aspartic acid) (PASP) have great potential for pharmaceutical and biomedical applications. In this study, we aimed to synthesize semi-interpenetrating PAAc/PASP hydrogels with improved swelling–deswelling properties via two-step polymerization, in which the first step of polymerization was performed at 37 °C for 15 min and the second step, the freezing polymerization, was performed at −20 °C for 24 h. The synthesized hydrogels were characterized with field emission scanning electron microscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis. The swelling and deswelling behaviors of the hydrogels in response to the ionic strength of the buffer solution were investigated. The Schott's swelling kinetic model was used to elucidate the swelling behavior of the hydrogels. The swelling and deswelling rates of the hydrogels prepared via freezing polymerization were faster than those of the hydrogels prepared via conventional polymerization. This was attributed to the large mean pore size of the freeze-polymerized hydrogels. The PAAc/PASP hydrogels that underwent freezing polymerization had better swelling–deswelling characteristics than the PAAc hydrogels. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2016