A H-bonded network in a 1:1 cocrystal of phenazine and meso-1,2-diphenyl-1,2-ethanediol places phenazine and diol in such a way that a thermally reversible photochromism is produced. The structure of the cocrystal is detd. by x-ray crystallog. Thermally reversible changes in the UV/Vis and IR spectrum on irradn., in combination with the generation of paramagnetism (ESR) lead to the interpretation that a light-induced electron transfer between 2 phenazine mols. is followed by a double proton transfer to the N atoms of the radical anion of phenazine.