Summary Three physical test methods for drying oils have been described. They are: Molecular weight determinations in cyclohexane for thermally polymerized and mechanically oxidized oils. Results have been compared with those obtained in benzene. Theoretical and factual reasons have been given for the apparent validity of average molecular weight determinations in cyclohexane. Dielectric constant determinations for treated oils. It has been shown that the dielectric increases as an oil is oxidized but that no increase in dielectric occurs on thermal polymerization. The dielectric constant is mainly a function of the oxygenated structure of the oil. Power factor determinations. The power factor increases on polymerization and on oxidation. The values obtained for any given viscosity are considerably greater for the oxidized samples. Molecular weight and dielectric constant determinations have been used to advantage in our plants and laboratories. Our knowledge on power factor determinations, however, is as yet not extensive but it is felt that with more background data on this constant a potentially meaningful tool for oil evaluation can be developed.