PURPOSE: To make precise detection of the end point being more improved than the conventional method possible by a method wherein a mixture for interfacial polymn. reaction of an arom. polycarbonate is irradiated with electromagnetic rays with a specified wave length and the ratio of incident rays to be absorbed and scattered throughout the course of the polycondensation reaction is monitored.

CONSTITUTION: In interfacial polymn. reaction of arom. polycarbonates, a sample of a polymer preparing reaction mixture for polycarbonate polymn. reaction which is a heterogeneous mixture comprising water, a water-insoluble org. liq., an organobishydroxy compd. or an organochloroformate oligomer, an acid acceptor, an effective amt. of polycarbonate interfacial condensation catalyst and arbitrarily, a polycarbonate chain terminator and phosgene is irradiated with electromagnetic rays with a wave length with at least about 300 nm and the ratio of the incident rays to be absorbed and scattered by the reaction mixture throughout the course of the polycondensation reaction is monitored until it reaches a specified limiting ratio at the end point of the reaction or in the vicinity thereof to detect the titled end point.