Abstract: The behaviour of the two kinds of the largest impurity centres of different sizes in emulsion grains under the action of oxidizing solution is studied by a method of development kinetics. The results indicate that, when acted upon by an oxidizing solution, the emulsion grains bearing larger impurity centres show a lawer development rate and the grains carrying smaller centres show a higher development rate but lose much of their ability to form latent image. Both of these phenomena are attributed to the destruction of the two kinds of the impurity centres by the oxidizing solution. The reason for the enhanced development rate of grains carrying smaller centres is supposed to be the loss of protection of the grains by gelation after the impurity centres being destructed. The results are much the same in the cases of unsensitized, sulphur-sensitized and sulphur-plus-gold-sensitized emulsion.