We report on magneto‐optical determination of the magnetic phase transition as well as the magnetic structure of crystalline CS3MnCl5. A new type of optical cryostat, with a closed cycle of He3has allowed us to study continuously the variation of the absorption bands down to 0.4 K. The temperature variation of hot and cold bands attributed to the 6A1→4E,4T2transitions proves that antiferromagnetic ordering arises at 0.6 K. Studying the MCD spectrum of the6A1→4T2zero phonon line in low magnetic field, we have observed that the antiferromagnetic phase reverts to paramagnetic state. From this we deduce a value of the weak exchange field (0.9 T) and the energy of the magnon (&ohgr;0= 0.28 cm−1). Using the latter value we show that the origin lines of the 4E and4T2states can be fairly well understood in terms of magnon sidebands. In contrast the &sgr; polarized absorption spectra under an applied magnetic field, perpendicular to the c axis, show no discontinuity. A log‐log plot of the absorbance versus magnetic field gives clear evidence for a power dependence with an exponent close to two, as expected when the spins are all orthogonal to the direction of the field.