We investigated photoisomerization dynamics of spironaphthooxazine (SNO) and its chloro-substituted derivative (CI-SNO) in crystalline phase by femtosecond diffuse reflectance absorption spectroscopy. Although the photoinduced ring-opening reaction occurred in 3 ps and <1 ps for SNO and Cl-SNO, respectively, photoisomerization to transplanar photomerocyanine did not take place. The generated non-planar open form of SNO lived for 2 ns, while that of Cl-SNO returned to the spiro form in I ps. The crystalline phase results are compared to the analogous experiments in amorphous solids with a view of restricted conformational changes of molecules embedded in a crystal lattice. (C) 2002 Elsevier Science B.V. All rights reserved.