Protons in liquid phases are stabilized by long-range electrostatic interactions, hydrogen bonding, and the formation of covalent bonds between H+ and solvent molecules. Thus, a small proton affinity, a low dielectric constant, or the inability to form hydrogen bonds that characterize many nonaqueous solvents hinders the transfer of protons from an aqueous phase. As a result, the particle that is readily transferred is a hydrated proton, H2n+1On+, rather than the bare proton, H+. Here we present calculations of the free energy of proton transfer from water to Equid acetonitrile, including the dehydrated particle, H+, and two hydrated particles, H3O+ and H9O4+- We use a combination of ab initio density functional, theory and a polarizable continuum model within the self-consistent reaction field method. This allows the first and second solvation shells of the proton to be described explicitly from first principles. Vibrational contributions to the enthalpy and entropy have been added in. Values taken from experiment are used for the vaporization free energies of water and acetonitrile. Our calculations suggest that the particle that is readily transferred is H9O4+. The model that best describes the transfer energetics consists of H9O4+ plus several acetonitrile molecules treated explicitly. For these models, the calculated and observed transfer free energies agree within 50 kJ/mol. Conversely, calculations for H+ or H3O+ lead to transfer energies that are too high. In most H9OP4+ models, the proton remains as a H3O+ species that coordinates to a first shell of water molecules and a second shell of solvent molecules hydrogen bonded to the water shell. The connection of these results with the current views of hydrated protons in polar environments, such as membrane proteins, is also discussed.