Secondary electron spectra for proton impact on water in the vapor and liquid phase are calculated and compared. Fast but non-relativistic protons are considered (∼0.3–10 MeV) where the first Born approximation is well justified. For both phases the response of the valence electrons is modeled according to the theory of Bethe by using available optical data, while the binary-encounter approximation was used for the K-shell electrons. In particular, for the vapor, photo-ionization data were used to construct the continuous optical-oscillator strength of the molecule and to evaluate the leading term of Bethe’s asymptotic expansion. The contribution of subsequent terms was established semi-empirically, allowing for the high-energy free-electron limit. For the liquid, the energy and wave number dependence dielectric-response function, which accounts for long-range effects in the condensed phase, was employed. Optical data and the impulse approximation were used to construct the full Bethe surface of liquid water and calculate cross-sections. The comparison was also extended to more simple (semi-) classical and optical models.