AbstractPhotolysis of 4‐R‐1,4(or 3,4)‐dihydropyrimidines causes ring contraction into imidazoles, provided that the substituent R is capable of sufficiently stabilizing an anionic centre (R = 2‐thiazolyl, 2‐pyridyl, 4‐pyridyl). Chemical evidence is presented to show that the ring contraction of 6‐phenyl‐4‐(2‐pyridyl)1,4(or 3,4)‐dihydropyrimidine occurs via heterolytic fission of the C(1)‐C(6) bond of intermediate 1‐phenyl‐6‐(2‐pyridyl)‐2,4‐diazabicyclo[3.1.0]hex‐2(or 3)‐ene. The anion‐stabilizing effect of R is correlated with the acid strength (pKa) of R–CH3. A pKavalue around 30 probably determines the border‐line between ring contraction into an imidazole and formation of