Poly(4-acrylamidosalicylic acid) gels exhibited multiple-phase behavior depending on their histories in the parameter space of pH and temperature. Four different phases, denoted as phase091 (as-prepared), phase244 (swollen at high pH), phase064 (heat-treated), and phase233 (swollen at high pH after heat treatment) were clearly resolved, where the three digits denote their linear swelling ratios in percentage with respect to their sizes at preparation. Each phase was stable and did not change its swelling ratio with pH or temperature as long as the values of pH or temperature were within limited ranges. Transitions among different phases were discrete with hysteresis loops. The structure factors corresponding to these four phases were obtained by small-angle neutron scattering, which indicated the presence of characteristic structures depending on pH and temperature, particularly in the shrunken state (i.e., phase064).