Self-excited flow oscillations in vaneless radial diffusers have been investigated experimentally to determine the effects of Reynolds number and diffuser geometry on the spatial and temporal characteristics of the phenomena. The ranges of Reynolds number, diffuser width to inlet diameter ratio and diffuser outlet to inlet diameter ratio covered in the present study are 3.5 x 10 - 6.5 x 10 , 0.037-0.074 and 1.55-1.84 respectively. The spatial distribution and frequency of the pressure fluctuations associated with the flow instability were measured by dynamic pressure transducers. In addition, time-averaged static pressure measurements were taken at various locations to characterize the mean flow in the diffuser. It was found that both the steady and the fluctuating pressures are the same on both walls of the diffuser at the same radial and angular positions. In the radial direction, the amplitude and phase of the fluctuating pressure however, varied in the radial direction but only the phase was found to vary in the tangential direction. These variations indicated that the phenomena is nonaxisymmetric with a curved wave front in the radial direction. The results also show that both the diffuser diameter ratio and width to inlet diameter ratio have significant influences on the non-dimensional frequency, fluctuating pressure coefficient of flow instability and static pressure coefficient, while the Reynolds number, in the considered range, has minor influence. For a diameter ratio of 1.55 the non-dimensional frequency and fluctuating pressure coefficient decreased with increasing width to inlet diamter ratio. The trends of the fluctuating pressure coefficient were reversed while the trends of the non-dimensional frequency were the same for a diffuser with diameter ratio of 1.84. The experimental data of the present work were compared with their corresponding theoretical data and were found to be significantly different. Analysis of the experimental data points towards the need to consider three dimensional flow effects in the diffuser in any new attempts to predict the phenomena.