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 via a chemically bonded form and the effect of the composition rate (polyurethane-acrylate) of the block copolymer were studied. In general, it was observed that the photoresponse (photomechanical effect) for the spiropyranes and, especially, the bis-spiropyranes is influenced by a combination of several factors, primarily, the spatial and electronic differences between the two forms (Sp.-Mer.) in the photoisomerization of these compounds. Such factors are discussed here.