Positron lifetime spectra were measured for several low molecular weight organic compounds, normal and cyclic hydrocarbons and their perfluorinated versions, from room temperature down to about 40 K, and information about the vacancies in them has been extracted from the lifetime and intensity of the ortho-positronium (o-Ps) component. Normally the sizes of vacancies were larger in the solids comprising larger molecules, showing that o-Ps can represent the vacancy size. In the special case of perfluorocyclohexane, the solid of which had to be prepared by sublimation, the vacancy size was larger than expected. In all of the solid molecules studied, two o-Ps states could be observed and their relative importance could easily change, showing a hysteresis-like behavior. At lower temperatures of around 40 K the o-Ps state with longer lifetime and larger intensity became overwhelming for all of the molecules. A further remarkable observation is that o-Ps lifetime and intensity were not very sensitive to most of the phase transitions, including the melting points.