The photochromic effect of polyurethane-acrylate block copolymers containing 6＇-nitro spiropyranes and 6＇nitro-bis-spiropyranes has been investigated. The influence of incorporation of the photochromic agent into the polymeric matrix as a simple solid solution or in a chemically bonded form and the effects of the composition (polyurethane-acrylate) of the block copolymer were studied. In general, it was observed that the photoresponse for the spiropyranes and especially the bis-spiropyranes obey the combination of several factors, mainly the spatial and electronic differences between the two forms (Sp.-Mer.) in the photoisomerization of these compounds. Such factors are discussed here.