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In this paper the design of compensators for output feedback systems which satisfy a sensitivity, reduction criterion is considered for the case when the dynamic compensator is an observer. Using the conditions for comparison sensitivity design of output feedback systems derived by Naeije and Bosgra , it is shown that for arbitrary stable state-observers (full or reduced order) there exist feedback gains multiplying the system output and observer states for which the system is stable and the sensitivity reduction criterion is satisfied. Use of observers enables direct control over some of the feedback system eigenvalues and leads to a useful interpretation of the sensitivity weighting matrix. A design procedure is described and illustrated by an example of an aircraft control. The results are graphically compared with results obtained by a conventional design procedure.