The structural and electrical properties of the Dion–Jacobson series layer perovskite HCa 2 Nb 3 O 10 were investigated. Within the intermediate temperature range (200–475 °C), the compound undergoes topochemical dehydration to Ca 4 Nb 6 O 19 and, under reducing atmospheres, partial reduction of Nb(V) to Nb(IV). These changes occur upon heating and are not reversed on cooling. Analysis of impedance data shows that the conductivity of Ca 4 Nb 6 O 19 is predominantly electronic under reducing atmospheres, consistent with the behavior of other structurally related mixed-valence layered niobates.