The character of nonexponential kinetics of the low-temperature decay of triplet carbenes in polymers has been given consideration. The temperature dependencies of the decay rate and the kinetic isotope effect were found at 77-180 K. The controlling factor in the kinetic nonequivalence of carbenes is the distribution of distances of the hydrogen atom tunneling from the matrix molecules. The effect of magnetic field on the rate of the carbene decay has been seen at 77 K. On the basis of the magnetic dependence view, the mechanism of the carbene decay involving spin and chemical dynamics of intermediate short-lived radical pairs has been advanced, The nature of magnetic effect in polymers shows the configuration inhomogeneity of radical pairs.