In this paper we present the results of a theoretical study of non adiabatic unimolecular dissociation processes with applications to the decomposition of N2O(1Σ) to yield N2(1Σ9) and O(3P). Such unimolecular reactions which involve a change in the electronic state can be handled by the theory of thermally excited intramolecular radiâtionless decay processes in analogy to molecular predissociation and electronic relaxation in the statistical limit. General criteria were advanced for describing the decay probability of a single vibronic level in terms of Fermi's golden rule and for specifying the (high pressure) unimolecular rate constant in terms of a thermally averaged transition probability. The quantum mechanical rate constant for the non adiabatic reaction is characterized by a pre-exponential factor determined by the interstate coupling matrix element and by a temperature dependent activation energy. At low temperatures the activation energy is equal to the continuum onset, and the reaction involves a tunnelling process. In the high temperature limit a general demonstration of the Franck Condon principle for thermal reactions was provided, whereupon the non radiative transition occurs at the intersection of the potential surfaces. Numerical calculations for a one dimensional model system for the thermal decomposition of N2O were performed utilizing the semiclassical approximation and confirm our general conclusions. A two dimensional linear model has been developed representing the rate constant in terms of a convolution of two generalized line shape functions, which enabled us to study the distribution of vibrational energy among the diatomic N2 molecules resulting from the thermal decomposition of N2O. Some predictions concerning the determination of single level decay probabilities and vibrational distribution of the molecular products are presented.