The formation of anodic films of thiourea has been investigated by means of current-time transients using the potential step technique. The transients can be theoretically well described with a transition from 2D to 3D nucleation (Stranski-Krastanov mechanism). Here, the formation of the first monolayer corresponds to a progressive 2D nucleation and growth with constant rate. After completion of this layer, 3D nucleation with diffusion controlled growth proceeds. At the noncovered regions of the mercury surface and of the first monolayer an oxidation reaction takes place leading to soluble products.