||The paper refers to the behaviour of a vaneless diffuser of a radial flow pump in partial flow operating conditions. Experimental data have been obtained within the diffuser using 2D/3C high repetition rate PIV coupled with unsteady pressure transducers. The pressure measurements were acquired on the shroud wall of the vaneless diffuser and on the suction pipe of the pump. Pressure signals were analyzed through frequency analysis and time frequency analysis. PIV results were post processed using original averaging methods. Two partial flow rates are investigated in this paper. For both flow rates, rotating instability occurs. For the lowest one, only one instability mode is developing whereas, for the other flow rates, two competitive modes do exist intermittently. Their spatiotemporal characteristics are described in the paper.