The absorption and MCD spectra of sodium isolated in a xenon matrix were measured between 22000 and 16000 cm 1 m the region of the free-atom 3s 2S 1/2 → 3p 2P 1, 2.3·2 transition. For the most stable “blue triplet” an analysis of moments was carried out giving values for the g-factor (g ??? = 0.15) and the spin-orbit splitting (Δ = 10 cm ??? of the exerted state. Our analysis shows that non-cubic vibrations of the lattice are largely responsible for the observed splitting: thus our results support the Jahn-Teller model for the triplet structure observed in the absorption spectra.