The aim of this study is the optimal dimensioning of the air circuit's outlet nozzle in relation with the load duration curve, for a given PEMFC generator, in order to maximize the PEMFC efficiency and to increase the net outlet power. The steady state PEMFC operation has been taken into account. The model of the PEMFC system used in the work is based on a moving least squares technique. A centrifugal compressor has been taken into account, and the operating line of the compressor has been evaluated for an optimal fixed opening of the outlet nozzle. A multi-level optimization procedure has been implemented to solve the optimization problem. The developed algorithm is useful to design an optimum air subsystem, reducing the number of the control variables and the consequences of the dynamic behavior of a controlled electric adjustable valve on the PEMFC performance. The results of the work can contribute to the improvement of the PEMFC generator reliability and of its cost/performance ratio. (C) 2010 Elsevier Ltd. All rights reserved.