Intersystem crossing (ISC) in pentacene dissolved in a single-crystal matrix of benzoic acid is shown to be dominated by near-resonant transfer of the excitation between the pure electronic SI state and vibrational levels of a higher triplet state. According to CNDOM calculations the triplet state is T-3, with the same symmetry as S-1 (pi pi* B-2u). The vibrational levels are assigned to fundamentals of out-of-plane modes involving motions of the hydrogen atoms, so that large changes in the ISC rates are observed when the vibrational frequencies are shifted by partial deuteration of pentacene. Normal mode calculations for different isotopic species of pentacene locate the fundamental frequencies of these out-of-plane vibrations in the range of 300-1100 cm(-1). The separation of the pure electronic S-1 and T-3 states must therefore lie in the same range, in agreement with the CNDOM calculations. For small energy separations of the singlet S-1 state and the vibronic triplet level, crossings between them can be induced by applying a magnetic field. These level crossings produce changes of the ISC rates and, as a consequence, changes of the fluorescence emission quantum yield. Well-defined level crossing signals have been observed at 1.6 K by monitoring the fluorescence intensity as a function of the Strength of the applied field. A quantitative analysis of these signals leads to a determination of the spin-orbit coupling matrix elements between the singlet and the triplet sublevels (in the range of 0.1 cm(-1)). The width of the resonances reflects the width of the triplet vibronic level, which at 1.6 K, is aetermined by its decay time of about 1.7 ps. When, at higher temperatures, pure dephasing dominates the population decay rates, the resonance effects are predicted to be diminished. In the experimentally accessible range of T < 15 K the onset of this evolution can be observed.