A photoreaction of caged calcium (Ca2+) was investigated by using the time-resolved transient grating (TG) method. The q(2)- dependent feature of the TG signal was analyzed based on a model that there are two parallel dissociation steps with different rates and Ca2+ is released promptly after the fracture of the caged compound. The TG signal representing the presence of Ca2+ was appeared by the volume contribution. Although the diffusion coefficients of Cg and the decomposed product are different without Ca2+, only one diffusion component was observed after the dissociation of CgCa(2+), indicating that the caged compounds and Ca2+ diffuse together. (C) 2008 Elsevier B. V. All rights reserved.