An adjustment mechanism (34, 36, 38, 40, 42, 46) for a stroke-adjustable reciprocating fluid machine (1) comprising a crank shaft (18) which is rotatable about a rotational axis (26), and a rotatable eccentric crank (22) eccentrically supported about the crank shaft (18) in a bearing (20), the crank (22) having a crank centre (24), and the crank centre (24) being adjustable relative to the rotational axis (26) between a largest and a smallest radial distance thereto, and an adjustment motor (10) connected to the drive (6, 14, 18, 22) of the fluid machine also being connected to the crank via a third angular gear (34) which cooperates with a fourth angular gear (36), the fourth angular gear (36) being connected to an adjustment output axle (38) having its centre on the rotational axis (26) and being connected to the crank by means of an adjustment sleeve (42), the crank shaft (18) being connected to the main drive shaft (6) of the fluid machine (1) via a first angular gear (14) which cooperates with a second angular gear (16).