IR spectrometry in combination with partial least square regression was used as an alternative method for detn. of the kinematic viscosity of a worn out engine oil at 100 degrees C by the std. method. A suitable spectral area was the 1262-654 cm(-1) without any math. adjustment of spectra. A very significant correlation was reached at R = 0.99 between the estimated values from the IR model and the lab. standardized values.