The eigenstructure assignment problem for a linear time invariant multi-input-multi-output system using output feedback is considered. A new approach is developed which identifies the eigenspaces for the desired set of all the closed-loop eigenvalues. For the assignment of this desired set, necessary and sufficient conditions are established. These conditions contain two coupled Sylvester matrix equations, one of which is proven to be a reduced-order square Sylvester matrix equation. This results in an efficient analytical procedure, numerically superior to known techniques, for the determination of the output feedback gain-matrix.