The deposition of positively charged latex particles with radii between 20 and 100 nm on a negatively charged mica surface is investigated with atomic force microscopy at low ionic strengths. The polystyrene latex spheres with amidine headgroups are characterized as to their size distribution, electrophoretic mobility, and aggregation behavior. Surface coverage measurements take into account the polydispersity of the particles on the surface. Deposition kinetics from a quiescent solution are, for low coverage, in good agreement with diffusion-limited adsorption. At long times, the surface coverage tends toward a maximum surface coverage theta(max) independent of the particle concentration but depending on ionic strength. This work extends the available data for theta(max) for small particle size and very low ionic strengths. The values for theta(max) decrease with decreasing ionic strength; this trend is described with a simple model based on random sequential adsorption and the effect of overlapping double layers. Particle size polydispersity can modify the results for theta(max) substantially; its influence is investigated in detail.