Optoelectronic properties of nanoporous Ge (np-Ge) have been investigated by Surface Photovoltage Spectroscopy. Electronic transitions in np-Ge have been compared with the ones obtained on crystalline and amorphous Ge, their dependence on ion implantation fluence and annealing treatment has been investigated. Np-Ge layers decorated with Au nanoparticles have been studied, and a significant photo-voltage enhancement, probably related to light trapping effects, has been found. This result can be of major interest for future photovoltaic applications.