In this paper we study the working space of a six-revolute decoupled robot manipulator. A simple and direct method is presented to obtain the boundaries of the total and primary workspace. The technique is based on finding the limit configurations of the general geometry positioning mechanism of the decoupled manipulator. In order to do this we derive a fourth-order displacement equation in the first joint variable. It is shown that the method only requires the simultaneous solution of two second-order nonlinear equations.