Naphthalene, which is excited into the lowest triplet state by unpolarized UV-Iight shows a strong deviation from the Boltzman distribution within the three magnetic sublevels, which is observed by ESR. We call this deviation optical spin polarization. In the mixed crystal C10D8: 0.2% C10H8 , where the undeuterated Naphthalene acts as trap at 4.2 °K, the optical spin polarization exceeds the thermal spin polarization such that a steady state inversion between two of the magnetic sublevels appears. This results in an emissive ESR-transition (stimulated emission of micro- waves). The time dependence of the spin polarization after switching on or off the UV-light is described by a superposition of two exponential functions with different rate constants. - The optical spin polarization is due to spin selection rules for the intersystem crossing process (ISC). These involve primarily the molecular symmetry. - The experimental results of the present paper are these selection rules for the pumping processes: The metastable triplet state is populated via the spin component τx.The depopulation rate constants of the three spin components τx, τv and τz are kx = 0.65 sec-1, ky=0.4 sec-1 and kz=O.15 sec-1 .The spin lattice relaxation probabilities are of the order of 1 sec-1. The latter are dependent on the direction of the magnetic field and on the concentration of the populated triplet states. - These results are compared with a theory of ISC.