Abstract The cis- and trans- [RhCl2(dmap)2]+ (dmap=1,3-bis(dimethylarsino)propane) complexes were prepared by reaction of RhCl3·3H2O with dmap, and the molecular structures of these complexes were determined by the single crystal X-ray diffraction method. Crystal data and final R values are: trans-[RhCl2(dmap)2]ClO4 (1), monoclinic, Cc, a=20.018(4), b=10.229(1), c=12.972(2) Å, β=99.98(2)°, V=2616(1) Å3, Z=4, R=0.034 for 1298 observed unique reflections. cis-[RhCl2(dmap)2]PF6 (2a), orthorhombic, Pca21, a=14.699(2), b=9.381(2), c=19.640(9) Å, V=2704(1) Å3, Z=4, R=0.086 for 1284 reflections. cis-[RhCl2(dmap)2]CF3SO3 (2b), orthorhombic, Pca21, a=14.700(3), b=9.381(4), c=19.644(8) Å, V=2709(2) Å3, Z=4, R=0.088 for 1094 reflections. The Rh–As distances in the linear As–Rh–As groups are 2.442(4)–2.454(4) Å (av 2.446(4) Å) for (1), 2.345(4) and 2.484(6) (av 2.415(6) Å) for (2a), and 2.389(10) and 2.441(10) Å (av 2.415(10) Å) for (2b), while those in the linear As–Rh–Cl groups are 2.385(3) and 2.370(4) Å (av 2.378(4) Å) for (2a), and 2.387(4) and 2.367(5) Å (av 2.377(5) Å) for (2b). The molecular structure of trans-[CoCl2(dmap)2]Clo4 (3) was also determined by the X-ray method. Crystal data and the final R value are: orthorhombic, Pccm, a=8.018(2), b=8.428(1), c=18.716(3) Å, V=1265 Å3, Z=2, R=0.095 for 1072 reflections. A series of trans-[RhX2(dmap)2]+ (X=Cl, Br, I) were prepared and their absorption spectra were compared with one another and with those of related complexes.