The formation and annealing out of paramagnetic defects in high-pressure and high-temperature diamond are investigated through a comprehensive heat treatment study of as-grown crystals. Diamond crystals are subjected to a sequence of isochronal anneals in the temperature range between 1550 and 2000 °C and the concentrations of paramagnetic defects are determined by electron paramagnetic resonance measurements after each annealing step. The quantitative data are analysed in the light of recently published results on the behaviour of nickel and/or nitrogen defects upon annealing. Like the P1 and W8 centres, the NIRIM1 nickel-related defects show a strong decrease in their concentration upon annealing at 1900 °C, whereas the NIRIM2 centres seem to be rather stable. The evolution of the nickel-related centre concentration through the annealing sequence shows that not all W8 defects transform into paramagnetic nickel–nitrogen complexes. Indeed, most of the W8 defects are transformed in other non-paramagnetic centres, with the AB1, AB3, and AB6 defects being probably by-products of this process. The relatively low concentration of nickel–nitrogen paramagnetic complexes formed upon annealing shows that their formation does not play any fundamental role in the kinetics of the nitrogen aggregation during heat treatments.