Abstract A new technique for analyzing thin film growth processes, called positron tunneling microscopy (PTM), is proposed as an extension of the recently developed positron reemission microscope. The unique feature of a PTM is that image contrast is provided by the exponential reemission probability for positrons tunneling through thin-film overlayers that present an energy barrier to reemission. Results of positron tunneling experiments show that PTM should have monolayer thickness resolution to processes that locally affect either the tunneling barrier's width, such as islanding and subsurface roughness, or the barrier's energy, such as lattice strain in pseudomorphic growth and compositional mixing in interdiffusion alloying. In the case of these latter effects where there may be no topological contrasts at all, experimental results are discussed in greater detail. Comparisons of PTM with existing electron microscopies are presented where appropriate.