The CIDNP effects observed in photolytic reactions of (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (TMBDPO) and benzoylphosphonic acid dimethyl ester (BDMP) in organic solvents of different viscosities are determined by S-T- transitions that occur in the region of energy levels crossing. An analytical expression is proposed for calculation of the CIDNP effects at weak magnetic fields, which takes into account the S-T- transitions due to energy levels crossing. The experimental data are compared to the results obtained by the numerical solution of the stochastic Liouville equation (SLE) for a microreactor model. The parameters that affect the efficiency of the S-T- transitions are discussed.