Photoreactive Langmuir-Blodgett films were made of poly(vinyl octanal acetal-co-vinyl cinnamate) (P(VO-VC)), and the effect of photo-cross-linking on the stability of a layered structure against thermal and chemical treatments has been demonstrated by energy transfer and surface plasmon spectroscopy. Photochemical cross-linking in P(VO-VC) layers easily occurred by UV irradiation for a few minutes, and consequently, the polymer became insoluble in a good solvent, dichloromethane. The durability against the solvent was measured by both UV absorption and surface plasmon resonance. A clear contrast between the irradiated part and nonirradiated part was seen in the picture taken by a surface plasmon microscope. The curing effect on the thermal relaxation of the layered structure was studied by the energy transfer method, which is very sensitive to changes in the layer distances. These findings indicate that photochemical cross-linking of the LB film greatly improves the thermal stability without any serious disordering of the layered structure.