Abstract: The formation of the two kinds of the largest impurity centres, of different sizes in emulsion grains was investigated by a method based on development kinetics on a pure silver bromide emulsion. The results show that under normal chemical sensitization and exposure conditions the smaller centres in the emulsion grains are formed primarily in the process of chemical ripening, whilst the major of the larger ones formed during exposure. The compatibility of the concept with some major kinetic features of chemical ripening and exposure was discussed.