In the last years we have started a spectroscopic survey in the visible of dark primitive asteroids. These objects, considered the remnants of primordial processes of our solar system, belong essentially to the taxonomical classes C, P, and D. They are thought to contain water ice in their interiors and mineral assemblages derived from aqueous alteration processes of surface materials. In this paper we present the observations of 18 low-albedo asteroids. Only one asteroid, 238 Hypatia, has shown a feature centered around 0.6μm, probably due to charge transfer transition Fe2+→Fe3+ in iron oxides (aqueous alteration products). The results obtained are discussed. The behavior of the reflectance of the objects with the heliocentric distance, with the albedo, and with the reflectivity of the known cometary nuclei will also be analyzed.