In this work, FTIR spectroscopy was applied to the determination of crystalline content in an aliphatic polyester. To this goal, a range of wavelengths (from 560 cm-1 to 680 cm-1), non conventional for this kind of polymers, was selected by analyzing spectra collected during isothermal crystallization tests. A deconvolution of the IR spectrum in that range showed the presence of three peaks sensitive to the amorphous content and one sensitive to the crystal content. An analysis of the time evolution of the absorbances allowed to determine the parameters needed to determine the absolute crystallinity degree. The time evolution of crystallinity during isothermal tests at different temperatures was successfully compared with results obtained by DSC. The procedure was also favorably compared with the result obtained by WAXD on a solid sample at room temperature.