Cyclones are widely used to separate particles from gas flows and as air emission control devices. Their cost of operation is proportional to the fan energy required to overcome their pressure drop. Evases or exit diffusers potentially could reduce exit pressure losses without affecting collection efficiency. Three rectangular evases and a radial evase with a variable opening were tested on two cyclones. Pressure drop was recorded for inlet velocities from about 10 to 20 m s(-1). The radial evase reduced cyclone pressure drop by between 8.7 and 11.9 percent when its exit area was equal to the flow area of the cyclone vortex finder or gas exit. A simple payback based on avoided energy costs was estimated to be between 3600 and 5000 h, not including installation cost. Published by Elsevier B.V.