Ruby red polycrystalline samples of 1-(4’-biphenylyl)-2-phenyldiazopropane (la) efficiently transform into colorless (Z)-1-(4’-biphenylyl)-2-phenylpropene [(Z)-3a] in up to 96% yield in a highly selective photochemical solid-to-solid reaction. The solid state reaction was shown to proceed via a carbene intermediate, and it was formulated asa 1,2-H shift with unprecedented stereoselectivity. It was shown that irradiation of la in inert solvents gives products via 1,2-H shifts [(Z)-3a and (E)-3a] and 1,2-Ph migrations [(Z)-4a and (E)-4a] in yields that vary strongly with the polarity of solvent. Irradiation of la in ethanol gave similar products along with ethers 5a from insertion of the carbene into the EtO-H bond.