The multichannel Raman spectrometry has been used in the study of the isomerization reaction of an 80% cis PA film into a trans PA, using a laser beam for a double purpose. It is employed simultaneously as an activation agent inducing the isomerization reaction and the Raman diffusion. In each experience, the power of the laser beam P-i(lambda) was equivalent to the temperature. Twelve spectra have been recorded at different time periods tj = j.dt. The integrations of the Raman intensities related to two selected bands were numerically calculated. We also proposed an original method for the determination of the isomeric composition. A quantitative relationship between the equilibrium temperature and the laser beam power (in the range of laser power: 30 < P-i(lambda) < 300 mW) has been found. An estimate number of isomerized molecules No and then a correction factors f(cis) and f(trans) were also obtained. (C) 2007 Elsevier B.V. All rights reserved.