SynopsisThermal history of a polyethylene-welded assembly has been determined by measuring the crystallinity via absorbance ratio (ROD) measurements. Fourier transform infrared microspectrometry has been used because it allows a space-localized analysis within a diameter of 200 pm.A linear relation between ROD and local crystallinity has been obtained using reference samples for calibration. Samples were calibrated by differential thermal analysis and density measurements. This relationship and the simulation of the various thermal treatments by calorimetric analysis allow us to characterize the different phases of the thermal history during the welding experiment.