After a discussion on the impact of chemical structure on the separation of small molecules on porous gels, the elution is described of forty hydrocarbons from cyclohexane, methylene chloride and isopropanol on vinyl acetate gel Merckogel OR-500. Different linear relationships of log molecular volume versus elution volume are shown to exist for different types of hydrocarbons, depending on the structure of the compounds and the nature of the vehicle. For certain structures the exclusion effect is masked by other effects. Experiments on model compounds indicate that there exists a relationship between the speed of the vehicle and zone broadening. This leads to the conclusion that the speed of the vehicle is dependent on the effects of axial dispersion, mass transfer, eddy diffusion and time dependence of equilibrium. Zone width is shown to increase with sample size. To demonstrate the possibilities of application some crude oils and aromatic parts of vacuum cuts are investigated by means of analysis by liquid chromatography. From the distribution curves recorded petroleum samples may be characterised.