A nondestructive ultrasonic procedure has been developed for evaluating in situ the relative deterioration of monumental stones. Ultrasonic waves are injected into the rock and they are registered, processed, and petrophysically interpreted after travelling through it. The procedure applied to a Spanish monastery, San Lorenzo de El Escorial in Madrid, built with a granitic rock, is described. The following methodology was applied: in situ categorization of the observable deterioration forms, sampling in the original quarries, laboratory aging tests and continuous ultrasonic monitoring, in situ data acquisition, evaluation of the ultrasonic parameters by signal processing, and stepwise discriminant analysis. All that information is petrophysically interpreted for an appropriate assignation of the in situ studied masonry blocks to the categorized deterioration degrees. Final in situ results from the Monastery of El Escorial are presented. Among the studied ultrasonic parameters, energy is the most useful of all and velocity the least.