The possible diffusion and localization of copper in hexagonal cordierite structure, either by Cu(I) insertion within the hexagonal channels or by Cu(II) substitution for Mg in octahedral sites have been studied. Limited amounts of Cu(I) (about 30% of theoretical values) can be inserted within the cordierite channels in statistically occupied 4e sites (0, 0, z with z = 0.06 and 0.10 respectively for Cu0.5Mg2Al4.5Si4.5O18 and CuMg2Al5Si4O18). Cu2+/Mg2+ substitution is limited by the competitive formation of mullite and spinel phases.