Non-exponential kinetics showing the induction period is observed for photoisomerization in solid phase. A possible source of the induction period is local minima along the pathway to the product which arises from the interaction of the isomerizing molecule with the surrounding molecules. In this case, multiple photoexcitations are required to complete photoisomerization. We show that product formation by photoisomerization in solid states can be analyzed by a simple equation (Eq. (10)). By fitting experimental data to this equation the number of local minima can be estimated. (C) 2010 Elsevier B.V. All rights reserved.