There is a world-wide emphasis on energy reducing technologies. Accounting accurately for the losses that arise from pipe fittings, such as valves, can result in cost and energy savings. Efficient designs, using (for example) correctly sized pumps, are only possible if reliable loss coefficient data are available. There are currently limited loss coefficient data available for predicting pressure losses in diaphragm valves for Newtonian fluids, and even fewer for nonNewtonian fluids. This is an urgent industrial problem, as the use of these valves is widespread in the mineral processing industries and consequent oversizing of pumps is common practice. In this paper, pressure losses were therefore measured for five Natco, rubber-lined, straight-through diaphragm valves of diameters ranging from 40 mm to 100 mm, using both Newtonian and nonNewtonian fluids for Reynolds numbers spanning laminar, transitional and turbulent flow. The measurements have confirmed that the diameter does affect the loss coefficient. Geometricand therefore dynamic-similarity does not exist for these valves. This is contrary to the data found in the literature where only one value is provided, with no indication of the valve diameter. It was shown that the non-Newtonian behaviour can be accounted for by using the appropriate Reynolds number. Empirical correlations were derived to calculate the loss coefficient for each valve for laminar, transitional and turbulent flow. In conclusion, the laminar flow loss coefficient constant provided by Hooper (1981) was confirmed and can be used for conservative designs. Accurate design, however, demands that the actual valve data should be used.