The use of non-invasive techniques enables to obtain data from built elements without altering them. This is very important in the case of heritage constructions, where the use of invasive techniques uses to be impossible. The technique of Ground Penetrating Radar (GPR), has been usually applied for the characterization of the subsoil, but nowadays is also a great tool for surveying the inner composition of masonry structures. These structures can be very thick, and the GPR enables to identify heterogeneities, cavities or singular elements inside. The investigation presents the assessment the inner composition of the pillars of the Gothic apse of Tortosa Cathedral (1345-1441). Initial studies were based on historical documentation and the stereotomy of the masonry, and determined that the pillars were solid and without infill. The results obtained with the GPR reveal differences in the internal composition of the several pillars. Thus, it is possible to define the existence of a heterogeneous core in the masonry pillars, which height is variable and related to the external stereotomy of the masonry and the constriction steps of the apse.