An internal combustion engine is described having a valve mechanism that comprises two cams 120,122 mounted coaxially and a summation lever 124 having cam followers 126, 127 in contact with both cams so as to move in proportion to the instantaneous sum of the lifts of the respective cams. A control spring 128 is provided to maintain contact between one cam profile and its respective follower(s), and a valve actuating rocker 114 having a stationary pivot and rotatably connected to the summation lever serves to open the engine valve 110 in dependence upon the movement of the summation lever, so as to enable the valve timing, valve lift and valve event duration to be adjusted by varying the phases of the two cams. In the invention, the summation lever is constructed in two parts 124a, 124b that can be selectively locked and unlocked to allow the valve lift to be deactivated and the motion of both parts is controlled by the control spring 128 when the two parts of the summation lever are unlocked from one another.