Polycrystalline perovskite-type CaMn1-xNbxO3 phases (with x=0.02,0.05,0.08 and 0.10) were investigated with regard to their structure, microstructure and thermal stability as a function of temperature. The studied phases revealed a complex microstructure at room temperature with 90° twinned domains. At high temperatures, the manganate phases underwent a structural transition from orthorhombic to cubic symmetry, as confirmed by in situ high-temperature X-ray powder diffraction and electron diffraction data. Thermogravimetric heating/cooling studies showed a reversible thermal reduction/reoxidation process that occurred above a defined transition temperature. A possible mechanism relating the high-temperature structural transition and the thermal reduction process of slightly substituted CaMnO3 phases was proposed. The thermal reduction process resulted in a change in the Mn3+/Mn4+ concentrations in the Mn sublattice, and therefore in a modification of the transport properties. A comprehensive study examined the impact of both phenomena on the electrical and thermal transport properties.