The use of mixed molecular crystals at low temperatures as systems in which to study laser induced photochemistry is discussed. As specific examples the photochemistry of s-tetrazine and its dimethyl derivative is treated. These molecules have previously been used as guests in host molecular crystals to illustrate solid state laser isotope separation and photochemical hole burning. The results presented here indicate that the low temperature tetrazine photochemistry may involve the sequential absorption of two photons. With this in mind, we are able to suggest a useful method of finding other materials for isotope separation and photochemical hole burning.