Polymerization process of poly(ether ketone) made of diphenyl ether (DPE) and isophthaloyl chloride (IPC) catalyzed by aluminum chloride in dichloroethane was characterized by H-1 NMR. The polymerization gave a precipitate of polymer/catalyst complex during the reaction. End-group analyses for polymer appeared as precipitate and oligomers stayed in the solution were carried out respectively after decomposition of the complex by treatment with methanol. Time dependence of the content of end groups showed that the oligomer of which molecular weight exceeded a critical value precipitated out independent of the functionality of the end groups, and further reaction proceeded in the precipitate as well as in the solution. Another approach to understand the reaction was carried out by direct H-1 NMR observation of the polymerization in an NMR sample tube. This experiment detected the molecular weight change of soluble oligomer in the reaction system at real time, and the results were consistent with those of isolated oligomers described above.