Co-substituted manganese oxide nanosheets, (Mn1-xCox)O-2 have been synthesized in the form of a colloidal suspension via a simple one-pot method. Substitution effects on the structural, optical absorption, and magnetic properties are investigated for the nanosheets and their lamellar aggregates. The composition of the (Mn1-xCox)O-2 nanosheets can be controlled continuously by adjusting the molar ratio of the starting materials. The solubility limit is x similar to 0.20 based on the cell volume. In the 0.00 <= x <= 0.20 range, the band gap energy, magnetic moment, and Weiss temperature change systematically with x. The charge density of the (Mn,Co)O-2 layer is independent of x (i.e., [(Mn,Co)O-2](0.2-)) and the cobalt ions are trivalent in low-spin state. (c) 2012 Elsevier Ltd. All rights reserved.