Experimental work was conducted at the Mount Isa Mines Limited lead/zinc preconcentrator to determine the relationship between ore particle size and dense medium cyclone partition characteristics. The cyclone partition characteristics, separation density and ecart probable, were objectively determined using a data smoothing technique which combined mass balancing and efficiency curve fitting in one step. The lognormal distribution was used to characterise the feed ore density distribution. Cyclone separation efficiency was expressed by an "s" shaped, reduced efficiency curve. Separation density and ecart probable were found to increase rapidly below a particle size of between 1mm and 2mm. Above 2mm almost no size effect was apparent. The data suggest that the deterioration with size was less severe at low feed medium densities. According to a theoretical analysis of the separation process, this could be attributable to lower medium viscosities. The density of the cyclone underflow medium pulp was always greater than that of the feed by an amount which decreased as the feed medium density increased. This phenomenon was attributed to medium solids sedimentation. Classification of the medium solids was almost negligible. No relationship was observed between cyclone density differentials and coarse ore (+2111111) ecart probables. The latter were approximately 40kg.m-3 at all feed medium densities. Coarse ore separation densities were closely related to the density of the underflow medium pulp. An empirical model was developed to examine the possibility of preconcentrating ore as fine as 0.6mm. The inferior separation characteristics of fine ore invariably lead to its removal during feed preparation. At Mount Isa the feed preparation cut size, 1.7rrm, causes almost 30% of the plant feed to bypass preconcentration. Through the construction of metal recovery/rejection rate curves, the likely success of preconcentrating 0.6mm ore was seen to depend greatly on the extent to which viscosity affected the separation characteristics of -2mm ore. More data are required to examine this aspect of dense medium separation.