An investigation of the spreading of emulsion latex particles on water is described. When the particles are spread with the aid of organic liquids, one particle thick surface films are formed. These films are best characterized by their greater thickness in comparison with conventional monolayer films. Equations are developed to calculate an average diameter of the spread particles from the projected area of an equal mass of monodisperse hard spheres of the same density and the experimental limiting area obtained from the pressure-area isotherm. The limiting areas of four polystyrene latexes studied are 114, 147, 292, and 365 cm 2/mg; the calculated average diameters are 138, 107, 54, and 43 mμ, respectively. The average diameters as obtained by electron microscopic techniques are: 154, 110, 66, and 46 mμ, respectively.