Kinetics of the delayed formation of latent image specks in vacuum was studied. It was assumed that the reaction of delayed formation of developable specks was the agglutination of one single silver atom species and one latent sub-image speck in a grain, and a theoretical equation for the increase with time of the fraction of developable grains was derived by use of the stochastic process theory as follows. _??_Here, P is the fraction of developable grains, t is the time of storing in vacuum after exposure, P ₀ is the P value at t =0, l and m are the number of single silver atom species and latent sub-image specks in a grain respectively, Δ P_l, _m is the fraction of grains which have l single silver atom species and m latent sub-image specks, and k ₁, ₁ is the reaction probability per unit time in such a grain that has one single silver atom species and one latent sub-image speck. The equation indicates that the reaction rate is larger when there are more silver specks in a grain. The experimental result was that the rate of increase of P was larger with larger exposures and at higher intensities. This is explained well by the above equation. The reaction probabilities k ₁, ₁ at high and low intensities were almost the same and were estimated at 0.10-0.11min^-1.