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Hydration of sulfuric acid playsa key role in new-particle formationin the atmosphere. It has been recently proposed that proton dynamicsis crucial in the stabilization of these clusters. One key questionis how water molecules mediate proton transfer from sulfuric acid,and hence how the deprotonation state of the acid molecule behavesas a function concentration. We address the proton transfer in aqueoussulfuric acid with O K edge and S L edge core-excitation spectra recordedusing inelastic X-ray scattering and with ab initiomolecular dynamics simulations in the concentration range of 0–18.0M. Throughout this range, we quantify the acid–water interactionwith atomic resolution. Our simulations show that the number of donatedhydrogen bonds per Owaterincreases from1.9 to 2.5 when concentration increases from 0 to 18.0 M, in agreementwith a rapid disappearance of the pre-edge feature in the O K edgespectrum. The simulations also suggest that for 1.5 M sulfuric acidSO42–is most abundant and that its concentrationfalls monotonously with increasing concentration. Moreover, the fractionof HSO4–peaks at ∼12 M. [ABSTRACT FROM AUTHOR]