Structures of the isotype phases of Cr3Si2, V3Si2, Nb3Si2, and Ta3Si2(characteristic space system D2d11) were studied. The formula detd. by x-ray anal. corresponded to M5Si3. Calcd. lattice consts. a and c for the silicides in kX were, resp.: 9.410, 4.747, V; 9.998, 5.062, Nb; and 9.86, 5.05, Ta. In the systems Nb-Si and Ta-Si, isotype phases were found, which can be considered high- or low-temp. modifications. Rotating-crystal and Weissenberg patterns of single crystals indicated tetragonal, space-centered elementary cells with lattice consts. a - 6.557, c 11.860 for Nb5Si3, and a = 6.503, c = 11.849 for Ta5Si3. The systems stabilized by C, V5Si3Nb5Si3 and V5Si3-Zr5Si3, with D88 structures were studied. Plots of cell-vol. against mole % indicated that the former gave a continuous solid soln., whereas the latter is not completely miscible. VSi2 and NbSi2 (C 40 type) were completely miscible in the solid state. In the system VSi2MoSi2 (type C40-C11), MoSi2 was very sol. in VSi2, but only small amts. of VSi2 dissolved in MoSi2.