Abstract: A motion compensation and tension control system (MCATS) for a surface vessel overboarding, umbilical cable supported, submersible payload is provided with a housing containing a pair of spaced parallel guide rods attached to oppositely disposed housing end plates. Two identical carriages, slidably disposed on the pair of guide rods, each include two parallel slide tubes rigidly attached by a cross plate and by an axle, with a plurality of sheaves rotatably supported by each axle. Motion of each carriage as a rigid body along the guide rods is elastically restrained by, either extension or compression springs. The surface vessel umbilical cable is received through an opening in one of the end plates, reeved around the two sets of sheaves, and mechanically connected to the other end plate. Wave induced surface vessel motion is transmitted through the umbilical cable and absorbed, or compensated for, by the spring restrained carriage motion before it reaches the submersible payload.