Novel acid-sensitive compounds comprising a cyclic ortho-ester that is acid-sensitive, and their salts, and comprising at least one hydrophilic substituent. These compounds are useful, for example, for forming conjugates (liposomes, complexes, nanoparticles and the like) with biologically active substances and releasing them into cellular tissues or compartments whose pH is acidic, or as nonionic surfactant for stabilizing particles encapsulating a biologically active substance and then destabilizing them in acid medium, or alternatively as a vector covalently linked to a therapeutic molecule so as to release said therapeutic molecule into the cellular tissues or compartments whose pH is acidic.