Single particle optical sensing (SPOS) is a highly sensitive particle characterization technique for the measurement of the large particle (>0.5 mu m) tail of a submicron particle size distribution. We have examined the SPOS technique for fumed silica dispersions which have a mass mean particle size of similar to 140 nm. The reported large particle population varies as a highly non-linear function of sample concentration and is accompanied by the distortion of the particle size distribution. A similar response is also demonstrated for monodisperse polystyrene latex spheres. The source of spurious particle counts and the distortion of the particle size distribution is shown to be secondary coincidence. Model calculations indicate that the primary contributors to spurious counts are particles near the peak of the size distribution. These particles are substantially smaller than the nominal minimum 0.5 mu m diameter. These findings establish the limited range of reliability for SPOS measurements when used to measure a small fraction of the total particle size distribution and establish the need for new particle characterization metrologies.