In this paper it is demonstrated that short chain alkanes (pentane, hexane, heptane, octane) can be adsorbed from a saturated vapour phase at the water–air interface. Measurements with the drop profile analysis tensiometry demonstrate that this molecular adsorption transfers into a condensation leading to a thin alkane film at the drop surface. At sufficient amounts, i.e. after such a film has reached a respective thickness, the condensed liquid starts to drain and an oil lens of alkane is formed at the water drop apex. The formation of such oil lenses is visually observed and leads to the situation that the obtained surface tensions are only effective values because the Laplace equation of capillarity does no longer describe the profile of the drop/oil lens. This is clearly demonstrated by the standard deviation of the profile fitting and the distribution of the deviation between experimental and calculated profile.