Magnetic circular dichroism (MCD) and absorption spectra are reported over the temperature range ~ 1.3–25 K and at magnetic fields up to 5 T for the (0, 0) band of the A2S+ ¬ X2Pi system of the mercapto radical isolated in Ar, Kr and Xe matrices. A simple model is presented in which the temperature and magnetic-field dependences of the MCD are ascribed to population differences between the levels of the X2Pi term that are split by the simultaneous action of spin-orbit and crystal-field effects. This model reproduces the observed positive sign of the MCD B and C terms. However, attempts to extend it to a quantitative interpretation of the data yield poor fits and unrealistic parameters, possibly as a consequence of strong guest-host binding.