A relatively simple, reliable means is described for measuring particle diffusivity in a particlefluid system in the core of a duct. Particles are injected into the midstream of the duct from a line source positioned normal to the direction of fluid flow. Particle diffusivity is calculated from measurements of the standard deviations of the particle concentration profiles at various downstream distances from the injector. At an air velocity of 10 ft./sec. under room conditions, 1μ oil droplets are found to exhibit a diffusivity of about half that of air alone under the same conditions.