Summary: Main topic of the paper is a problem of designing the input-output decoupling controllers for nonholonomic mobile manipulators. We propose a selection of output functions in a general form. Regularity conditions guaranteeing the existence of the input-output decoupling control law are presented. Theoretical considerations are illustrated with simulations for mobile manipulator consisting of RTR robotic arm mounted atop of a unicycle which moves in 3D-space.