AbstractViscous interaction may prevent or retard diffusional deposition of fluid‐borne particles and droplets onto a collecting surface. The possible importance of viscous interaction has been recognized in the literature(1–3), but quantitative results which demonstrate the importance of viscous interaction are scant. Rather, the scant reported evidence is confusing. This paper reports the results of a theoretical investigation of the influence of viscous interaction on diffusional deposition of particles. The results show negligible influence in gaseous media, but sometimes significant retardation in liquid media, especially if electrical double layers also retard deposit