In a compressor particularly of an exhaust gas turbocharger of an internal combustion engine having a compressor wheel rotatably supported in a compressor housing which includes a diffuser structure with an adjustable diffuser geometry comprising guide vanes pivotally supported, an adjustment ring is provided with engagement elements which engage forked end structures of the guide vanes at the outer ends of the guide vanes opposite the pivotal guide vane support so that the pivot position of the guide vanes can be adjusted by rotation of the adjustment ring.