A reciprocating piston mechanism (1) comprises a crankcase (15) and a crankshaft (2) which has at least a crankpin (4). The crankshaft (2) is supported by the crankcase (15) and rotatable with respect thereto about a crankshaft axis (5). The mechanism further comprises at least a connecting rod (9) including a big end (8) and a small end (10), a piston (11) which is rotatably connected to the small end (10), and a crank member (6) which is rotatably mounted on the crankpin (4). The crank member (6) comprises at least a bearing portion (7) and has an outer circumferential wall which bears the big end (8) of the connecting rod (9) such that the connecting rod (9) is rotatably mounted on the bearing portion (7) of the crank member (6) via the big end (8). The crank member (6) is provided with a crank member gear (12). The crank member gear (12) is an external gear, which meshes with at least an intermediate gear (13). The intermediate gear (13) is an external gear, which also meshes with an auxiliary gear (14). The auxiliary gear (14) is an external gear, which is fixed to an auxiliary shaft (16) that extends concentrically through the crankshaft (2). The crankshaft (2) and the auxiliary shaft (16) are rotatable with respect to each other.