To evaluate the effect of soil water content, clay type, and clay content upon the capillary-diffusion of liquid water in unsaturated soil, and to attempt to establish a functional relationship between capillary-diffusion and self-diffusion of liquid water in unsaturated soil, capillary-diffusion coefficients were measured by use of inflow and outflow methods. With both methods the capillary-diffusion coefficients decreased very rapidly with decreasing water content. Self-diffusion coefficients were measured over a moisture content range from air dryness to saturation using tritium as a tracer. (Author)