Abstract The 1 H, 13 C and 19 F NMR spectra of photochromic p-substituted 2, 4, 4, 6- tetraphenyl-1, 4-dihydropyridines IIa-IIg, 1-methyl-2, 4, 4, 6-tetraphenyl-1, 4-dihydropyridines IIIa-IIIg, 2, 4, 4, 6-tetraphenyl-4H-pyrans IVa-IVh, and 2, 4, 4, 6-tetraphenyl-4H-thiopyran V were inspected; it was found that compounds IIa-IIg occur in a dynamic equilibrium with their dihydro tautomer VIa-VIg. Also deuteriodeprotonation of IIa and IIIa and their reaction with ...