Experimental work has been undertaken to quantify the influence of angle of inclination on hydrocyclone performance in mineral processing applications. Testwork was undertaken at Mount Isa Mines on copper and lead-zinc slurries and in the JKMRC laboratory on a gold ore and other ores. In both cases the cyclone performance was monitored at a range of cyclone inclinations from zero to 112 degrees to the vertical. Cyclone diameter varied from 102 to 508mm. The results show that cyclone inclinations greater than 45 degrees can strongly affect the performance of industrial cyclones. A new empirical model of classifying hydrocyclone performance has been developed on the basis of this data, incorporating angle of inclination and slurry viscosity as variables, and including prediction of cut-size, separation quality, water recovery and the pressure-flowrate relationship.