CaCu3Ti4O12 (CCTO) ceramics have been processed by solid state reaction and sintered at 1100°C for different times. A clear increase of the dielectric constant of the material up to values of 6×104 has been observed with the sintering time. This increase is accompanied by a limited grain growth and intergranular Cu-oxide phase thickness reduction. The disappearance of the Cu-oxide phase is caused by the incorporation of Cu cations into the grains, contributing to the increase of the dielectric constant. Raman spectroscopy shows the decrease of TiO6 octahedral rotational modes with the sintering time due to the incorporation of Cu cations into the CCTO grains. XANES measurements show that the Cu main oxidation state is Cu2+ and does not change with the sintering time. The fitting of the experimental dielectric constant to the Internal Barrier Layer Capacitance (IBLC) model reveals the change of the intergranular phase dielectric constant, caused by a compositional change due to the incorporation of Cu into the CCTO grains.