The photodissociation of s-tetrazine (ST) and its dimethyl derivative (DMST) has been investigated in hexane solutions at room temperature and in argon matrices and organic molecular crystals at cryogenic temperatures. Contrary to what has generally been assumed, only ST photodissociates upon excitation into its lowest excited singlet state at room temperature and in the rare gas matrix. Both ST and DMST in molecular crystal hosts exhibit a quadratic dependence of photodissociation on exciting light intensity. This we consider to be evidence of a sequential two photon photodissociation process. © 1978.