Data from a range of filtration experiments on dilute particle fluid mixtures are used to determine the parameters that describe the physics of suspension flow in compaction. The range of solids volume fractions used is 0.00001<~0.1; <-potentials vary between O-50 mV. The relevant physical data are extracted from an analysis of the initial stages of a range of experiments at various - and - Theoretical considerations on suspension flow are presented to argue that the physical character of the flow at relatively dense, strongly interacting conditions is significantly different than that of quite dilute systems. The latter are dominated by fluctuations in the particle velocity near the septum to give gas-type diffusive btthaviour, while in the former the particles are more or less locahzed This observation has implications for the diffusion coefficient, which is predicted to behave quadratic in the filtration pressure for very dilute media and which is roughly independent of this quantity for mixtures containing strongly interacting particles. Experiments are described and analyzed to establish this behaviour and the experimental trends that are obtained bear out the main theoretical insights.