The electrical properties of a series of CaCu3Ti4O12 ceramics prepared by the mixed oxide route and sintered at 1115 degrees C in air for 1-24 h to produce different ceramic microstructures have been studied by Impedance Spectroscopy. As-fired ceramics are electrically heterogeneous, consisting of semiconducting grains and insulating grain boundaries, and can be modelled to a first approximation on an equivalent circuit based on two parallel RC elements connected in series. The grain boundary resistance and capacitance values vary as a function of sintering time and correlate with the ceramic microstructure based on the brickwork layer model for electroceramics. The large range of apparent high permittivity values for CaCu3Ti4O12 ceramics is therefore attributed to variations in ceramic microstructure. The grain-boundary resistance decreases by three to four orders of magnitude after heat treatment in N-2 at 800 degrees-1000 degrees C but can be recovered to the original value by heat treatment in O-2 at 1000 degrees C. The bulk resistivity decreases from similar to 80 to 30 Omega.cm with increasing sintering time but is independent of heat treatment in N-2 or O-2 at 800 degrees-1000 degrees C. The origin of the bulk semiconductivity is discussed and appears to be related to partial decomposition of CaCu3Ti4O12 at the high sintering temperatures required to form dense ceramics, and not to oxygen loss.