Abstract: A Scotch yoke mechanism for actuating opposed-piston gas compressor is disclosed. The mechanism comprises a crank pin eccentrically extending from an end of a main drive shaft, a slider rotatably surrounding the crank pin, and a yoke defining a pair of opposed parallel sliding surfaces for confining the motion of the slider only to the sliding motion along the surfaces, said yoke further defining a pair of outwardly extending connecting rods for connecting oppositely disposed pistons on either side of the york, whereby the orbital motion about the axis of the main drive shaft causes the slider to reciprocate the yoke and thus oppositely disposed pistons.