The feasibility of measuring refractive index (RI) on sub- milligram samples of artists' paint was assessed. Medium-rich paint, glaze and varnish samples are suitable for such testing, and their RI can be measured to ± 0. 005 in many cases. New and thermally-aged samples of 19th century oil/resin formulations for gelled media were examined. It was found that there was no increase in RI on ageing, while the presence of lead driers and pigment had a significant effect on the RI of both new and aged material. Modern, unpigmented paint media showed variously increased and decreased RI after thermal ageing. The increased RI expected from the increased transparency of oil paint as it ages was not found consistently in 19th century paintings. Instead, independently analysed glaze samples with high RI were resinous rather the comprising pure oil. The RI of paint seems to depend on its formulation more than on its age. The observed transparency of hundred-year-old paint may be due to complex pigment-medium interactions, some at least involving lead compounds. -- CAL