The electrocrystallization of MnO2 was studied on a disc-shaped Pt ultramicroelectrode at 50 and 80 degrees C. A recently developed model that accounts for the spherical diffusion towards a microdisc was employed. A non-linear fitting procedure between experimental data and theoretical equations revealed a progressive nucleation with a three-dimensional growth controlled by diffusion. The calculated rate of nuclei formation (AN(infinity)) varied between ca. 1 to 9 X 10(8) cm(-2) s(-1) at 50 degrees C and 1 to 30 X 10(8) cm(-2) s(-1) at 80 degrees C in the potential interval investigated. The diffusion coefficients were also obtained for 50 and 80 degrees C having values of 7.1 X 10(-6) cm(2) s(-1) and 8.6 X 10(-6) cm(2) s(-1), respectively. The linear dependence of AN(infinity) with potential was analysed in terms of the atomistic theory of electrocrystallization. This allowed the calculation of the number of molecules in the critical nucleus as being 3 at 50 degrees C and 6 at 80 degrees C. The shape and distribution of nuclei were investigated by scanning electron microscopy. The results showed that hemispheres of different diameters were formed on the surface, thus confirming the progressive nucleation and growth mechanism.