4-Acylhydroxypyrazoles combined with thiosemicarbazide derivants exhibit excellent photochromism in the crystalline state. Upon irradiation with 365 nm light, colorless enol-form isomers are converted to the yellow keto-form isomers. FT-IR, UV/vis, and fluorescence spectra were used to investigate the photochromic behavior of 1-phenyl-3-methyl-4-(3-chlorobenzal)-5-hydroxypyrazole 4-methylthiosemicarbazone. A possible photochromic mechanism has been proposed based on the analysis of the crystal structure of the sample.