Experimental studies were carried out on the caracteristics of diffusers of rectangular cross section which have an angle of expansion 2θ=30°, 60°, 90°, and 180° and an area ratio of 4. Remarkabel improvement of the pressure coefficient and energy loss coefficient is achieved by means of suction at the entrance. It is found that the most effective position of suction is at the point right down stream of the corner. The diffuser efficiency increases with increase of aspect ratio. The efficiency, however, tends to be constant as the aspect ratio exceeds four. When the angle of expansion is 20°, 30°, 60°, and 90°, the rate of optimum suction is 3 to 4 per cent of the total flow rate. With this optimum suction, the efficiency of these wide angle diffusers approaches that of the diffuser of 2θ=10°.