A direct method of determining the mean diameter of particles executing Brownian motion is presented. The temporal coherence of the scattered field from submicroscopic particles illuminated by laser light is a function of both the integration time and the particle diameter. The temporal degree of coherence of the time-averaged scattered intensity decreases as the integration time increases. Statistical processing of the scattered photons leads to a method of particle sizing (average diameter), which circumvents the need for digital autocor-relation or power spectral estimation.