The photochemical reaction of 2-nitrobenzaldehyde (NBA) in various electrolyte solutions as well as in a solution with a protein is investigated from the viewpoint of the diffusion coefficient by using the time-resolved transient grating (TG) method. For studying reaction with optically silent materials such as electrolytes, we found that the diffusion coefficient is useful for identifying chemical species involved in the reaction. The TG signal that represents the diffusion process was dramatically changed by adding electrolytes such as Cl- or Br-, and this result indicates that these electrolytes should participate in the photochemical reaction of NBA. It was found that the time profile of the grating signal also depends on the addition of guanidine-HCl or apomyoglobin. The structural change of the protein associated with the NBA reaction is discussed.