The isothermal crystallization of poly (phenylene sulfidesulfone) (PPSS) fromN‐methylpyrrolidone (NMP) solutions has been monitored by a viscoelastic technique. We have observed four characteristic changes which occur with increasing time in 20 wt % solution at 24 °C; (i) a decrease in η*, (ii) a steep increase inG’, (iii) a slight decrease in η*, and (iv) an increase in η* andG’. These changes indicate the crystallization process from a nucleation to an aggregation. Further, the crystallization at several temperatures and of poly (ethylene terephthalate) (PET), poly (butylene terephthalate) (PBT) melts was measured similarly. It was found that the rate of crystallization of PPSS from NMP solutions is very slow, and its temperature dependence is very small.