We have computed ab initio transition energies, equilibrium geometries, force constants and potential energy curves for the dissociation of S-0, T-1, and S-1 of two nitrosoalkanes, CH3NO and t-BuNO. A normal coordinate analysis has been performed for the three states, and the harmonic wave function for the C-N bond torsional coordinate has been replaced by hindered rotor eigenfunctions. The n --> pi* absorption spectra have been simulated by computing the appropriate Franck-Condon factors in order to assign the vibrational sub-bands. The predissociation lifetimes of several vibrational states of S-1 have been evaluated by computing nonadiabatic and spin-orbit couplings, which determine the Internal Conversion and Intersystem Crossing rates. For t-BuNO the computed lifetimes (10-160 ns) are in the same range as those measured by Noble [J. Chem. Phys. 85, 5763 (1986)]. The lifetimes of CH3NO, for which no experimental data are available, are longer (50-330 ns). Both the IC to S-0 and the ISC to T-1 are important.