Diamonds grown at higher temperatures than usual (about 1500 °C) contain nitrogen in both the A centre form (paired nitrogen) and the C centre form (isolated nitrogen). The distributions of A and C centres were measured in the {111} growth sectors of crystals grown from cobalt metal, using Fourier transform IR equipment fitted with a microscope, in order to investigate how the A centre is formed. The centres were found to be inhomogeneously distributed. In some regions more than 90% of the nitrogen was in the form of an A centre, whereas in other regions the A centres were less than 10%. In general, the concentration of the A centre was higher in an inner region than in an outer region. However, its concentration fluctuated along a growth direction and was not uniform even within a growth plane. The distribution suggests that the A centre is formed by aggregation of nitrogen from the C centre, but the aggregation rate is not constant even within a {111} sector.