A model is presented for materials that deform due to exposure to light. This model is based on the conclusions drawn from mechanochemistry that chemically induced ionizations cause changes in the dimensions of porous polyelectrolytic gels. The mechanophotochemical properties of poly[p-(N, N-dimethylamino)-N-γ-D-glutamanilide], that displays up to 35% dilation in each dimension when exposed to light, are presented as proof of the correctness of the model. © 1978, American Chemical Society. All rights reserved.