Caries, one of the most important oral diseases, is associated to a progressively demineralization process. The aim of the present work is the study of the dynamics of the artificial demineralization process of human dentin by means of a non destructive ultrasonic pulse-echo technique. Human dentin samples were sliced and artificially demineralized with phosphoric acid at 10%. The demineralization process was monitored using a 15-Mhz pulsed-echo transducer. The reduction of the stiffness of the dentin, and suggests that the acid demineralization, lasting about one hour, develops at a non-constant penetration rate. A model to study the diffusion of the acid into the dentine tissue is proposed to evaluate demineralization lesions produced by natural caries in-vivo.