An account is given of the use of the optical Zeeman effect in determining the zero-field splitting parameters and the signs of spin-orbit interaction matrix elements for triplet states of organic molecules. The results for pyrazine, pyrimidine and s-triazine are used to exemplify the effects. These experiments involve high resolution studies of the spectra of single crystals at 4.2 K in polarized light and in magnetic fields ranging from 6 to 5 kG.