The large complex of Hospital de la Santa Creu i de Sant Pau in Barcelona, built between 1901 and 1930 and designed (in their largest part) by architect Lluís Domènech i Montaner, constitute one of the major examples of the Catalan Modernism which flourished in Barcelona in the early 20th century. The historical part of the complex was inscribed as UNESCO World Heritage Site in 1997. In addition to their interesting architectural and aesthetic value, the buildings draw attention because of their genuine structural features. The structures combine a significant amount of steel members with masonry ones. In most of the pavilions of the complex, masonry is utilised for walls and single- and double-curvature Catalan vaults forming the floor slabs and ceilings, while steel members provide the main vertical structure sustaining the vaults as well as the ties necessary to retain their horizontal thrust. The combination of steel and thin tile vaults permits diaphanous and functional spaces, both wide and tall, with large windows allowing satisfactory levels of natural light. A large part of the complex is at present being subjected to detailed studies and restoration works for the rehabilitation and adaptation to new uses. The buildings have required a deep repair due to the corrosion of steel members. The paper comprises two parts. Firstly, the structure of one of the pavilions, Nra. Sra de la Mercé pavilion, is described with focus on the vault morphology and the combined use of steel and masonry members. Secondly, a tentative structural analysis is presented on two different selected vaults of the same pavilion. The analysis carried out, aimed to provide an estimation of the loading capacity, takes into account the combination of masonry and steel members that compose the vaulting system.