We applied scanning force microscopy to study, in real time, the mobility and reorganisation of single poly(2-vinylpyridine) molecules that were adsorbed on mica and exposed to vapours of ethanol or water. The macromolecules adopted a compact globular conformation in ethanol-saturated vapour and extended to a surface-bound two-dimensional coil in water-saturated vapour. Hence, reversible coil-to-globule conformational transitions of flexible polymer chains were directly visualized with unique molecular resolution.