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.