From reactions of equimolar quantities of the heavier lanthanoids (M = Gd – Lu) with the elements sulfur and iodine in fused silica ampoules (900 °C, 10 d) phase-pure samples of the lanthanoid sulfide iodides MSI (M = Gd – Lu) could be prepared. Besides the already structurally determined sulfide iodides MSI with M = Gd and Dy, all samples could be characterized by means of X-ray powder diffraction and their magnetic behavior was examined. In addition a single-crystal structure refinement for HoSI was carried out (orthorhombic, Pmmn; a = 415.09(4) pm, b = 531.34(5) pm, c = 920.53(9) pm), which confirmed the isotypy of the later lanthanoid sulfide iodides with the structure of FeOCl. On the basis of the X-ray powder diffraction experiments all prepared compounds could be indexed in a primitive orthorhombic unit cell. An analysis of the determined lattice parameters and cell volumes resulted in the typical lanthanoid contraction for compounds within the row of the 4f elements. The magnetochemical studies led to the expected effective magnetic moments for trivalent lanthanoid cations without showing any magnetic ordering phenomenon.