The Mn-substituted CuInS2 compounds (Culn(1-x)Mn(x)S(2) with x = 0-0.20 and Cu1-yMnyInS2 with y = 0.05 -0.10) were synthesized using high-temperature solid-state reactions. Single-phase materials with the chalcopyrite structure persist with up to 10% of the Cu/In sites being replaced with Mn. The introduction of manganese results in a linear expansion of the lattice parameters as a function of Mn concentration, following Vegard's law. Rietveld refinements on a combination of X-ray and neutron powder diffraction data reveal a site preference of Mn for the In site under In-poor conditions and the existence of cation anti-site occupation, Cu-In and In-Cu. The Mn substitution increases the anion displacement, accentuating the cation-anion bond length mismatch. The greater variance in the bond alternation and the addition of Mn d-S p hybridization near the Fermi level result in a decrease in the bandgap by similar to 0.1 eV. The CuInS2:Mn compounds display paramagnetic behavior with short-range antiferromagnetic interactions. X-ray photoelectron spectroscopy suggests the presence of Cu+, Mn2+, and In3+ in the samples. (c) 2012 Elsevier B.V. All rights reserved.