Transition state theory is extended straightforwardly to treat nonadiabatic processes and applied to study the rate constant of the spin-forbidden reaction CH(2Π)+N2→HCN+N(4S). A one-dimensional model was set up to calculate the intersystem crossing probability with the distorted wave approximation and using an ab initio value of the spin–orbit coupling. The effect of orthogonal degrees of freedom was then considered by energy convolution with the vibrational frequencies, obtained from ab initio calculations, orthogonal to the crossing seam at the minimum of the seam of crossing (MSX), also obtained from ab initio calculations. An expression for the cumulative reaction probability, N(E), of the reaction was obtained by a straightforward extension of the unified statistical theory, where the MSX was treated as a transition state. The calculated thermal rate constant, k(T), seems to be too low by two orders of magnitude compared to experimental measurements and an empirical transition state study where empirical vibrational frequencies at the MSX are lower by a factor of 2 than those derived here. The disagreement strongly suggests that the current treatment of the multidimensional dynamics needs to be improved. In particular, it may be a poor assumption that spin-forbidden transition takes place with uniform probability on the seam in the case we are considering. © 1999 American Institute of Physics.