Low-speed tests have been made to investigate the performance of three annular diffusers having centre bodies of uniform diameter and conically diverging outer walls. The diffusers had a common area ratio of 2.0 : 1, an inlet radius ratio of 0.833, and non-dimensional lengths of 5.0, 7.5 and 10.0 respectively. The tests were carried out with fully developed flow at inlet; the inlet conditions were obtained by natural development in a long annular entry length.