This paper describes a fundamental study of evaporation of volatile-liquid lenses due to the heat supply from the substrate of an immiscible, less-volatile liquid under the atmosphere of the common vapours of the two fluid substances. Experiments were performed with single n-pentane lenses placed on the surface of an otherwise quiescent pool of water. The behaviour of each lens throughout its life was observed in detail using laser shadowgraphy. It was found that the time required for complete evaporation of each lens and the behaviour of the lenses change, in a peculiar but rather regular way, with the age of the water surface. This ageing effect is more significant when fluids of higher purities are used, and it is thought to be due to the quantity of contaminants at the surface increasing with its age.