The acid ionization of HCl in water is examined via a combination of electronic structure calculations with ab initio molecular orbital methods and Monte Carlo computer simulations, The following key features are taken into account in the modeling : the polarization of the electronic structure of the solute reaction system by the solvent, the quantum character of the proton nuclear motion, the solvent fluctuation and reorganization along with the solvent polarization effects on the proton potential, and a Grotthuss mechanism of the aqueous proton transfer, The mechanism is found to involve the following : first, a nearly activationless motion in a solvent coordinate, which is adiabatically followed by the quantum proton rather than tunneling, to produce a contact ion pair Cl--H3O+, which is stabilized by similar to 7 kcal/mol; second, motion in the sol vent with a small activation barrier, as a second adiabatic proton transfer produces a solvent-separated ion pair from the contact ion pair in a nearly thermoneutral process, Motion of a neighboring water molecule-to accommodate the change of the primary coordination number from 4 for H2O to 3 for H3O+ of a proton-accepting water molecule-is indicated as a key feature in the necessary solvent reorganizations. It is estimated, via a separate argument, that the remainder of the process to produce the completely separated ions involves a free energy change of less than 1 kcal/mol. It is argued that the reorganization of the heavy atoms between which the proton transfers plays an essential role in assisting the adiabatic (nontunneling) and stepwise transfer mechanism and that the concerted pathway of the multiple proton transfers in water is unfavorable.