In this work, we determined the number average molecular weight of some polyethylene glycols (PEGs) using vapor pressure osmometry. In general, this technique works with a plot of Delta r(p)/c(p) versus c(p) (concentration) and the molecular weight is determined by the calibration constant obtained from the curve intercept. We demonstrated that this method induces a high dispersion of the data, which can be minimized using a plot of Delta r(p) versus c(p). Therefore, more precise values of molecular weight can be obtained and the number average molecular weight, M-n, of several PEGs is directly determined from the ratio between the curve slopes obtained for a standard and the polymer sample.