A one-electron energy diagram is constructed for FeS 2, and its modification for CoS 2 and NiS 2 are discussed. A T- nl- b phase diagram is constructed, where nl gives the occupancy of the narrow σ∗ band determined by chemistry and b ∼ ∊λσ is proportional to the width of the narrow σ∗ band in the itinerant-electron domain. Here ∊ is a one-electron energy and λσ is the eg-( sp3) covalent-mixing parameter for σ-bonding orbitals. These two diagrams are used to interpret the varied magnetic properties of the systems Fe 1- xCo xS 2, Co 1- xNi xS 2, CoS 2- xSe x, and CoS 2- xAs x. In the mixed systems, the σ∗ bands must be separated into donor and acceptor bands, and it is shown how this may be done for each system. The ferromagnetic to antiferromagnetic transition in CoS 2- xSe x is attributed to the appearance of a spin-density wave in the narrow transitional range of b between full-moment ferromagnetism ( μCo = 1 μB) and Pauli paramagnetism.