Abstract: A composite sliding block to be arranged between two supporting plates of a frictional-type seismic isolator, with one supporting plate connected to the superstructure to be isolated and the other to the foundations, comprising two contact components that externally are slidingly or articulatedly in contact with said supporting plates, depending on whether the isolator has one or two sliding surfaces, and internally are coupled to each other by means of a male projection of one component in a female recess of the other. An elastomeric seal occupies an empty space surrounding the projection within the recess and, on the external side of the contact component(s) slidingly in contact with the supporting plate(s), a sliding plate is accommodated in a corresponding niche having an elastomeric compression support at the bottom. Frictional pendulum-type isolators with one or two concave sliding surfaces include such a block.