Three radiationless decay pathways for the photochemical decomposition of diazirine and diazomethane have been characterized using the MC-SCF method with a 6-31G(*) basis. From diazirine, two almost barrierless paths exist on S-1. One leads, via a diradicaloid D-1(sigma pi) conical intersection at a bent, in-plane, diazomethane-like structure, to ground-state diazomethane; the other leads, via another D-1(sigma sigma) conical intersection at a ring-opened diazirine diradicaloid geometry, directly to (CH2)-C-1 + N-2. The triplet pathway starts at a (3) pi-pi(*) diazirine minimum, passing over a 9 kcal mol(-1) barrier to a (3)n-pi(*) D-3(sigma pi) bent diazomethane-like minimum from which the barrier to N-2 extrustion is 7 kcal mol(-1). In the absence of sensitizers, this triplet path can be entered from the singlet manifold via intersystem crossing at a point that has been characterized by finding the lowest energy point on the singlet-triplet crossing surface. This crossing point occurs at a geometry that is very similar to the transition state that occurs on the singlet path between diazirine and ground-state diazomethane. However, the efficiency of intersystem crossing (spin-orbit coupling) is predicted to be low. These data rationalize the temperature dependence of the fluorescence, the fact that diazomethanes and diazirines are observed as products of photolysis of diazirines and diazomethanes, respectively, the fact that there is CH2 + N-2 formation from both diazirines and diazomethanes, and the fact that no triplet states seem to be involved in the reaction.