Experimental results for the electron spin resonance of the S− and Se− centers in alkali halide crystals are analysed and a theoretical model for their description is established. Most of the experimental results can be explained by assuming a strong T-t Jahn-Teller effect in the ground-state system combined with a spin-orbit coupling of similar strength. In particular, the principal g-tensor values, which are of importance for comparison with experiment are derived. They show good agreement with the measured data.