An experimental study was performed to investigate the mechanics of fluid flow in a 30<SUP>o</SUP> annular diffuser, and to study the periodicity of flow oscillations in this region. Surface oil-flow patterns and smoke-flow visualisation experiments were performed with a centrebody concentric and eccentric in the diffuser. Above a threshold offset, the measurements revealed two contra-rotating periodic vortices in antiphase with each other, symmetrically disposed about the plane of minimum clearance in the annulus, and originating in the 30<SUP>o</SUP> diffuser. Steady pressure measurements indicated that the steady fluid forces acting on the centre body are decentralising, and any small perturbation will result in the centrebody being pushed towards the wall of the diffuser. Unsteady pressure and force measurements showed that for a mean eccentric centrebody position, there was a predominantly tangential unsteady vortex force present with the centre body both fixed and vibrating. This vortex force scaled linearly with flow velocity, indicating a Strouhal-type mechanism. The magnitude of the vortex force was independent of both amplitude and frequency of vibration of the centrebody, indicating a forced vibration effect. The exception was when the centre body frequency approached the vortex shedding frequency, in which case lock-on occurred. For the geometry considered, lock-on does <i>not</i> significantly increase the unsteady forces acting on the centrebody. During lock-on it was found that the vortices could not only be influenced by centrebody motion, but could be completely suppressed by closely controlling the amplitude and frequency of the centrebody. The effect that shaking the centrebody has on the different flow regimes in which the annular diffuser operates is explained. The vortices could also be eliminted by a) attaching a small helical fence to the surface of the centrebody, and b) by inserting a perforated liner within, and downstream of, the diffuser section. A small perturbation theoretical analysis of the unsteady flow in the diffuser has been developed. The flow was computed numerically, and the predicted self-induced forces examined. The analysis predicted mainly negative damping for the configurations examined. The predicted magnitude of the unsteady forces agreed with experimental results. Finally, the flow was also predicted analytically, and a good level of agreement with the numerical study was found.