This paper describes an approach for the control of pressurized PEM fuel cell systems used in automotive traction applications. This model-based controller design approach is based on a 13-states non-linear dynamic model. The focus of the paper is in controlling the excess air ratio while tracking an optimum variable pressurization for maximum system efficiency during load transients. The control approach combines a feed-forward approach based on the steady state plant inverse response, coupled to a multi-variable LQR feedback controller. The controller shows excellent performance over severe load transients with both actual states and state observers feedback.