SAMP (subsystems and moving packets) is a recently developed method for modelling preferential flow and solute movement in porous media, and for determining the fate of "new" and "old" water and solute when the new enters a region containing the old. In a SAMP model, the modelled region (e.g. the unsaturated zone) is divided into cells, and the pore space within each cell is divided into many (often 100 or more) subsystems. Packets of water, containing solute, move within and between the cells, from subsystem to subsystem, thus simulating the bulk movement of water and solute. A theory is developed here for representing molecular diffusion in the liquid phase of porewater as interpacket diffusion, and this theory is implemented in the SAMP I one-dimensional vertical-column model. The model is found to exhibit appropriate sensitivity to its parameters, and is successfully calibrated against existing laboratory breakthrough data for tritium movement in Glendale silty clay loam. The quality of fit achieved to the laboratory data is found to be significantly better when interpacket diffusion is simulated than when it is not. The main parameters for the model are those for the matric potential and unsaturated hydraulic conductivity functions, and the only parameter requiring calibration is the internal scale, which affects both interpacket diffusion and the way packets move within the soil. Theoretical and numerical comparisons show there are similarities between the internal scale and the coefficient for solute exchange between the dynamic region and dead-space in the two-region (mobile-immobile) model of van Genuchten and Wierenga (1976).