The stability and decomposition behaviour of CaCu5-type CeCo5 + x compounds (0.1 less-than-or-equal-to x less-than-or-equal-to 1) at 650-degrees-C and 750-degrees-C have been investigated using optical microscopy, scanning electron microscopy, X-ray diffraction and electron probe microanalysis techniques. The homogeneity range of this CaCu5-type CeCo5 phase at 1000-1100-degrees-C has been assessed to be from 83.6 to 85.7 at.% Co (CeCo5.1-CeCo6). When the cobalt concentration is close to the lower limit of high temperature homogeneity range (83.6 at.% Co), only CeCo5-Ce5Co19 phase boundary precipitation of Ce2Co17 takes place; when the cobalt concentration is lower than the congruently melting composition (85.0 at.% Co), its decomposition behaviour is identical to that of the stoichiometric RCo5 compounds (R = Pr, Gd, Y). When the cobalt concentration is equal to or higher than 85.0 at.% Co, the decomposition behaviour of the grain boundary is essentially the same as that of the compound whose cobalt concentration is lower than 85.0 at.% Co, the intragranular decomposition is of the spinodal mode, and the parent phase (CeCo5.56 or CeCo6) decomposes into Ce2Co17 and isomorphous CeCo4.93.