In this dissertation, detailed calculations of multiple ionization cross sections of noble gases by intermediate-velocity protons are presented. A detailed study on the relative importance of the probabilities for both the direct ionization of electrons from a given sub-shell and the postcollisional emission has been performed. The cross sections obtained in this work have also been compared to the experimental data and theoretical results available in the literature. In order to investigate the influence of the adopted values for the probabilities of direct ionization of each target sub-shell, different versions of the general equations have been used, in which the participation of each sub-shell has been gradually neglected. The use of probabilities for postcollisional emission from different authors has also been analyzed. It was observed that, for higher final charge states, the multiple ionization cross sections become more and more sensitive to both the direct ionization and the post-collisions emission probabilities.