When dealing with the repair by grouting of masonry structures, the improvement of their carrying capacity has certainly to be considered as one of the most important aims of the intervention. Nevertheless the long term performance and durability of repairs has also to be taken into account; repairing should also insure an increasing of the service life of the structure. The sort carried out by the authors concerned three main purposes: -to detect the effectiveness, from the point of view of bond strength, of epoxy formulated resins used for injection. The experimental work based on mechanical tests and optical observations was performed on shear bond specimens; -to calculate and evaluate, also with the aid of non destructive tests, the improvement of the mechanical strength of decayed masonry prisms repaired by injection; -to test the durability of resins and of injected masonries to thermal cycles. To this purpose accelerated aging tests were performed on shear bond specimens.