Monomolecular films of a poly(vinyl alcohol) derivative having an azobenzene (Az) side chain (6Az10-PVA) transferred onto mica by the Langmuir-Blodgett technique were observed by atomic force microscopy in the noncontact mode. Multifarious features of the film morphology after the thermal and photostimulated processes were revealed, which were strongly dependent on the humidity of the atmosphere, the lateral packing density of the monolayer, and the light exposure dose. In a humidified atmosphere, a low-density film (1.2 nm(2) Az(-1)) showed a network morphology that exhibited a two-dimensional expansion upon irradiation with UV (365 nm) light (trans -->cis Az isomerization). The films having the higher densities (0.4 or 0.3 nm(2) Az(-1)) gave spongy-shaped or partially collapsed films and indicated a formation of three-dimensional protrusions on UV light illumination. In comparison with the spectroscopic data, these morphological changes could be related with the proceeding of the Az photoreaction. The morphological changes were basically reversible, however, and showed large deviations depending on the back (cis --> trans) processes, i.e., thermally or photochemically with visible light (436 nm). In-situ tapping mode observations of 6Az10-PVA monolayer on mica in water were further performed. The monolayers immersed in water showed completely different photoinduced deformations with no reversibility.