The T -> S-o phosphorescence of isolated and identically oriented quinoxaline molecules was studied in high magnetic fields. The intensity ratios of the three Zeeman components are a function of the field direction. They are used to determine the relative values for the radiative decay rate constants of the three zero field levels of the triplet state: k(z)(D)/k(x)(D) > 50, k(z)(D)/k(y)(D) > 50.