We present ellipsometric measurements on Langmuir-Blodgett monolayer assemblies of photochromic polymers; the polymer we used was a statistical copolymer with polymethacrylate mainchains and azobenzene sidegroups. Ellipsometry gives a value of n equals 1.557 for the refractive index and a film thickness of 1.58 nm per monolayer. When exposed to light of appropriate wavelength, the azobenzene groups undergo a trans (reversible reaction) cis photoisomerization process which was monitored by ellipsometry. We found an average change in film thickness of 0.02 nm per monolayer during the trans(reversible reaction)cis transition process, whereas the data obtained do not show any significant change in refractive index.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.