Brewster angle microscopic (BAM) observation was undertaken for a photosensitive monolayer composed of a poly(vinyl alcohol) derivative bearing an azobenzene (Az) side chain. BAM images revealed clear differences in the morphological and rheological properties between the trans and u.v.-light isomerized cis monolayers. Real time observation of the u.v. light induced expansion of this monolayer was successfully achieved. An as-spread domain structured monolayer was converted to a highly homogeneous one after a trans-->cis-->trans photochemical cycle.