We studied the nucleation of two-dimensional crystals from micrometer- and submicrometer-size latex particles in liquid films formed on mercury, glass, and mica. For the experiments on mercury we controlled both the film thickness and the mercury surface electric potential, whereas for the experiments on glass and mica we traced the film thickness as a function of time. The nucleation of monolayers from micrometer-size particles was obtained on mercury only in a narrow interval of low positive electric potentials applied to the mercury. Both the micrometer and the submicrometer particles started to assemble when the film thickness became equal to the particle diameter. Our results suggest that the lateral capillary immersion forces, which appeared between the particles protruding from the liquid film, arranged these particles in a two-dimensional-crystal nucleus.