Interpenetrating polymer networks (IPN) of poly(aspartic acid) and poly(acrylic acid) salts were produced by crosslinking of poly(acrylic acid) in the presence of K or NH4 salts of poly(aspartic acid) (PAs-K and PAs-A, resp.) in aq. soln. The pH of reaction mixts. had a high impact on the process. IPN of PAs-A and poly(acrylic acid) showed hydrogel-like properties and were recommended as water absorbing additives to mineral fertilizers.