This work describes a method to determine mechanical properties (tensile strength at break and tensile modulus) of poly(ethylene terephthalate) using median infrared spectroscopy and multivariate calibration. Infrared spectroscopy is very promising for polymer process control and final product analysis because it is rapid and nondestructive. The spectra of the films were obtained using two techniques: attenuated total reflection and direct transmission. The spectra were subjected to various preprocessing procedures, such as smoothing and derivative using the algorithm Savitzy-Golay, standard normal variate, multiplicative scatter correction and, as well, combinations of some of these preprocessing techniques. The predictive ability of the regression models were evaluated using an external validation set. The regression techniques used, partial least square and multiple linear regression, showed, in general, comparable results with root mean square error of prediction similar to the repeatability of the conventional method used to determine these mechanical properties (1.3 kgf/mm2 for tensile strength at break and 29.6 kgf/mm2 for tensile modulus). (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013