Photosensitive monolayers containing both an azobenzene and a spiropyran group were prepared by the Langmuir-Blodgett technique. These films displayed a good reproducibility and a high stability extending up to several months in the dark. Large circular domains with a diameter of several micrometers were seen about 3.5 nm above a homogeneous phase. From geometrical considerations and investigations with different ratios of molecules containing the spiropyran and the azobenzene groups, these domains were interpreted as bilayers of mainly spiropyran molecules. The photoisomerization of both the spiropyran and the azobenzene group was performed. After an initial irradiation with ultraviolet light, the spiropyran transformed to merocyanine and subsequently to J-aggregates. The morphological and the optical changes induced by light were investigated by atomic force microscopy and by absorption spectroscopy on the same layer. The growth of J-aggregates was found to be the best explanation for the observed roughening of the circular domains. In situ atomic force microscopy allowed us to follow the spectacular morphological changes as a function of time and of the irradiation. At the beginning, a small upper structure was seen, usually at the edge of a domain. This upper patch spread progressively within the domain. The resulting multibranched shapes were related to a diffusion-limited aggregation process. Ultraviolet and visible light were seen to induce the presence of nucleation points and to stimulate the roughening process. The morphological change of a domain could also proceed slowly in the dark after the photoinduced nucleation.