Ethyl 4-formyl-1,3-dimethylpyrazole-5-carboxylate showed coloration upon 366-nm light irradiation in the solid state at room temperature. The red color disappeared either by melting or dissolving into solvents. The initiation of coloration is most likely based on a photochemical intramolecular hydrogen abstraction by the formyl oxygen from the methyl group on C-3. It also exhibited coloration in 2-methyltetrahydrofuran matrix at low temperature.