Infrared spectroscopy has been coupled with the polarization modulationtechnique in order to characterize the molecular orientation in films ofazopolymers irradiated in situ with polarized visible light. The resultsobtained on disperse red 1-containing amorphous azopolymer (pDR1A) demonstratethe high efficiency of polarization modulation infrared linear dichroism todetermine quantitatively the time dependence of the orientation function ofseveral chemical groups during the orientation (laser on) and relaxation(laser off) processes. The difference dichroic spectra show that polarizedvisible light induces a preferential orientation of the azobenzene groupsperpendicular to the direction of the polarization of the writing laser. Thisorientation of the side chains also results in a slight cooperativeorientation of the C=O ester groups of the main chain of the polymer. The timedependence of the orientation function shows that the orientation andrelaxation processes could be described by a biexponential function involving"fast" and "slow" response modes. The rate constants associated with thesemodes are the same for the main chain and the side chains of the polymer.Finally, we have observed that the level of orientation and the rate constantsof the orientation and relaxation processes depend strongly on the filmthickness.