Abstract The effect of in situ and ex situ Cu doping on MgB 2 was studied in the annealing temperature range of 600 °C–900 °C. Attempts to substitute Cu into MgB 2 resulted in the formation of Mg–Cu phases. Within the limits of error, neither lattice strain nor significant alteration in a - and c -axes was introduced by varying annealing temperature. This indicates that all the samples showed negligible Cu substitution or Mg deficiency. Both in situ and ex situ Cu doping caused an increase of density in the samples annealed at 800 °C by 28% and 26%, respectively. SEM micrographs show that the in situ samples consist of larger grains with low Cu content as well as agglomerates of fine grains which correspond to Cu rich regions. On the other hand, compositional variation in the ex situ sample is less pronounced. The reaction temperature causes only minor changes in the T c onset but influences the superconducting volume fraction.