Experimentally there appears to be a limit to the size of isolated patterns of electrocatalytic nuclei below which electroless metal deposition does not occur. From a model of electroless deposition on small isolated circular spots we conclude that this inhibition effect is caused by an enhanced supply of oxygen to small patterns due to nonlinear diffusion of dissolved oxygen. Since generally the kinetically determined rate of oxygen reduction is larger than that of oxidation of the reducing agent, the open‐circuit potential of nuclei in small patterns cannot be shifted to a sufficiently negative value at which metal deposition is initiated. At relatively large substrates with a high density of small patterns this so‐called oxygen‐diffusion‐size effect may not be noticed since the supply of oxygen is limited by linear diffusion to an effectively planar substrate.