A diffuser for a radial flow compressor includes a pair of coaxial rings, the second of which closely surrounds the first. A set of intersecting flow passages extends through the first ring, and another aligned set of flow passages extends through the second ring. In a first embodiment of the invention, a circumferential groove is formed between the two rings, thus allowing low momentum fluid to migrate from the pressure side of one passage to the suction side of a neighboring passage and to prevent boundary layer build-up. In a second embodiment of the invention, the passages are arranged in a double cone configuration which allows higher diffusion levels than are attainable in a straight cone diffuser of equal length.