The flow field in the vaneless diffuser fitted to a centrifugal blower with self excited gas oscillations was investigated experimentally. Systematic measurements of steady and fluctuating flow velocity and pressure were undertaken in the diffuser. Nine different diffuser configurations with and without through flow were tested to determine the effect of diffuser geometry on the steady and unsteady flow field. Regions of inward reversed flow on both sides of the diffuser were observed to exist in all different conditions of no through flow measurements, while for the case of small through flow rate a reverse flow was observed on one side near the inlet of the diffuser. The oscillations with and without through flow were found to differ markedly in non dimensional frequency, wave form and amplitude. In both cases however the oscillations filled the entire diffuser space both radially and axially and their characteristics were found to be dependent on diffuser geometry and rotor rotational speed. The onset of the oscillation was found to be associated with a discontinuity in the overall machine performance curves. Both the pressure rise and flow coefficient were observed to be reduced.