An analytical methodology has been developed for the rapid and reliable determination of the hydrogenation rate of spent lubricating motor oils. Size exclusion chromatography (HPLC-SEC) combined with refractive index (RI) and diode array detection (DAD) was used to estimate qualitatively the changes in the composition of the spent lubricant oil with respect to the reaction time. The DAD signal of the segments of the effluent as they are separated in molecular size order provides a unique fingerprint of the unsaturates present in the oil as it portrays the features of their compositional analysis. The UV-DAD absorbance signal was expressed as a function of the molar mass, and it has been modeled as the sum of several molar mass Gaussian distributions. The compositional changes that occur during hydrogenation are accounted as changes of the heights of the Gaussians, and a simple model is proposed to predict the compositional changes of the chromophore groups present in the feed during a hydrofinishing process.