Post-tensioned composite slab systems consist of concrete slabs poured on thin-walled profiled sheeting and pre-stressed soon after casting. This paper presents a theoretical model aimed at evaluating the ultimate moment resistance of this form of construction. In the first part of the paper the key equations describing the model are introduced and these are then followed by the model validation carried out against experimental results available in the literature. The comparisons carried out between the experimental measurements and the calculated capacities show good agreement.