Charged particles travelling through a molecular material slow down by collisions with the molecules of the medium. The products of these interactions are either excited molecules or electron-hole pairs, i.e. pairs made of a free electron and the corresponding positive ion resulting from ionization. Additional pairs are produced in further interactions of the liberated electrons in a cascade process. The collection of all the liberated charges forms the basis of ionizing radiation detection in ionization chambers [1]. To determine the dose, conversion factors such as W-values are necessary. They are defined as the mean energy expended by the incident particle to form an electron-hole pair after complete dissipation of its initial energy in the media. This physical parameter represents an important source of uncertainties in hadrontherapy that influences directly the dose determination. The aim of the present work is to study W-values to improve uncertainties and provide values when experimental data are not available. To calculate this parameter, inelastic cross sections and the cumulative counting of all the processes induced by the incident and secondary particles are necessary. As the contribution of secondary electrons to the calculation is of crucial importance for all types of incident particles (electrons, ions, etc.), we started studying W-values by electron impact on liquid and vapour water. We have used two methods: i) the Monte Carlo code MDM [2] which does an event-by-event tracking of all generated particles, and ii) the Fowler Equation based on the Continuous Slowing Down Approximation [3]. The relevance of the cross sections used to describe the ionization and excitation processes is studied. Results are in good agreement with experimental data and theoretical values obtained by other authors. We are working to extend the codes to the case of ion impact on water and air, where charge exchange and multiple ionization must be considered. Preliminary results will be presented.