To date, all existing literature on the so-called ‘high permittivity’ perovskite oxide CaCu₃Ti₄O₁₂ (CCTO) in the form of ceramics, single crystals and thin films show the grains (bulk) to exhibit semiconductivity with room temperature, RT, resistivity of ∼10–100 Ω cm. Here we show that CCTO grains can be highly resistive with RT resistivity >1 GΩ cm when CCTO ceramics are processed at lower temperature (700 °C). With increasing processing temperature, the semiconducting CCTO phase commonly reported in the literature emerges from grain cores and grows at the expense of the insulating phase. For sintering temperatures of ∼1000–1100 °C, the grains are dominated by the semiconducting phase and the insulating phase exists only as a thin layer grain shell/grain boundary region. This electrical microstructure results in the formation of the so-called Internal Barrier Layer Capacitance (IBLC) or Maxwell–Wagner mechanism that produces the commonly reported high effective permittivity at radio frequencies in dense ceramics. The relationship between Cu loss at elevated processing temperatures and the transformation of the grain resistivity from an insulating to semiconducting state with increasing processing temperature is also discussed.