Visible light irradiation of thin films of a main-chain liquid crystalline poly(aryl cinnamate) using ketocoumarins as triplet sensitizers leads to photochemical crosslinking and W-vis and FTIR spectroscopic changes associated with saturation of the cinnamate double bond, most likely by 2 + 2 photocycloaddition. The triplet sensitizers are themselves photolabile and are lost by photochemical reactions during the sensitization process. A new ketocoumarin sensitizer with decyloxy substituents and a reduced tendency to phase separate from the polymer is reported. A simple calculation of the sensitization stoichiometry shows that a single molecule of this ketocoumarin sensitizes the destruction of approximately 90 cinnamate chromophores in the "as cast" films below T-g and about 300 chromophores in the more-ordered glassy nematic films and in "as cast" films of poly(vinyl cinnamate). Triplet sensitization of fluid nematic films leads, upon initial irradiation, to UV-vis hyperchromism that is attributed to disruption of chromophore aggregation and, possibly, to disruption of the nematic mesophase as photoproducts begin to form. (C) 2000 John Wiley & Sons, Inc.