The oxidation in cation deficient spinels of copper iron spinels CuxFe3-xO4 (0 < x less than or equal to 1) synthesized by soft chemistry with a grain size < 50 nm has been investigated by thermal measurements (DTG and DSC), For stoichiometric synthesized spinels when the copper content determines the number of oxidizable cations 1-x per mole of ferrite, three oxidation phenomena corresponding to Cu+ at B-sites(130 degrees C), Fe2+ at B-sites (180 degrees C) and Cu+ at A-sites (240 degrees C) have been found in close relation with the cation-oxygen distance of each oxidizable cation. For nonstoichiometric synthesized spinels CuxFe3-xO4+delta with delta < 0 when the oxidation reveals a mass gain larger than that calculated with 1-x, the presence of additional interstitial Cu ions has been envisaged, The oxidation temperature of these interstitial ions (> 300 degrees C) is higher than that for Cu+ ions at A-sites suggesting that interstitial copper ions also essentially reside in A-sites.