Data on the spectroscopy of photoexcitation of trans -to- cis isomerization and fragmentation of protonated trans -azobenzene (ABH + ) in an inert gas atmosphere are analyzed. Results from gas-phase isomerization are compared to ones from a liquid-phase reaction in aqueous sulfuric acid that results in the formation of cyclic benzoccinnoline. There is no cyclization upon gas-phase trans -to- cis isomerization in a rarefied nitrogen medium at T = 300 K, due to the high rate of inverse cis -to- trans isomerization. It is shown that transformations in both phases proceed with the participation of phenylaminyl type cations trans - and cis -ABH + , which are chromogens. Photofragmentation of trans -ABH + in an atmosphere of cryogenically cooled helium ( T = 40 K) proceeds with the formation of fragment phenyl and phenyl diazonium cations and the participation of phenyl amine cations. Mechanisms of photofragmentation are proposed, including intermediate acts with the participation of H atoms formed by dissociation of the NH group. A fast attack of H atoms along the phenyl ring bonded to the NH group results in the breaking of the C–N bond, eliminating the possibility of trans -to- cis isomerization in a cryogenic helium atmosphere.