We report CASSCF(6,6)/cc-pVDZ optimized geometries, energies (also single-point CASPT2(6,6)/cc-pVDZ), electron spin–spin dipolar interaction (D,E) tensor, and spin-orbit coupling (SOC) for m-xylylene in the lowest triplet T1 (13B2), in the next triplet 13A1, and in the slightly higher 23B2. The zero-field splitting (zfs) parameters computed for T1 (D/hc = 0.013 cm–1, E/hc = –0.003 cm–1) agree well with the observed values |D/hc| = 0.011 cm–1, |E/hc| < 0.001 cm–1. If 3A1 is the T2 state as calculated, its computed D/hc (–0.040 cm–1) and E/hc (0.001 cm–1) agree with the value |D/hc| = 0.04 ± 0.01 cm–1 deduced from experiment assuming E = 0. If 23B2 is the T2 state, the experimental data need to be reevaluated, since its computed E/hc value (–0.012 cm–1) is not negligible relative to D/hc (0.038 cm–1). The SOC matrix elements of T1–T3 with the lowest and the ππ* excited singlets are small (≈0.01–0.1 cm–1), while those with representative 1σπ* states are large (≈10 cm–1). The former lack one-center terms and therefore are much smaller than expected from the standard one-electron approximation. Computed SOC affects D and E slightly, and supports the proposed vibronic mechanism of intersystem crossing from T2.