The paper is focused on the development of a Kalman filter type observer for the monitoring of the primary drying phase of the freeze-drying of pharmaceutical products in vials. The proposed soft-sensor is able to estimate the evolution of temperature of the product, the duration of the primary drying phase, the value of the overall heat transfer coefficient between the heating shelf and the product, as well as the mass transfer resistance of the dried cake to vapor flow. Accurate results are obtained for various types of products, characterized by a different dependence of the mass transfer resistance on the dried cake thickness. Theoretical results are confirmed by experimental tests carried out in a pilot-scale freeze-dryer. Finally, the strength and the weakness of the proposed observer are pointed out. (C) 2011 Elsevier Ltd. All rights reserved.