[en] The ionic liquid–gas interface of 1-hexyl-3-methyl-imidazolium chloride, [HMIM][Cl], was examined in the presence of water vapor using lab-based ambient pressure x-ray photoelectron spectroscopy (APXPS) at room temperature. The interfacial water uptake was measured quantitatively in the pressure range of high vacuum up to a maximum of 5 Torr (27% RH) and back to high vacuum in a systematic manner. Water mole fractions in the interface determined from APXPS were compared to previously published tandem differential mobility analysis results on [HMIM][Cl] nanodroplets. Our findings show that water constitutes a significantly larger mole fraction at the interface when compared to the bulk. Additionally, the reverse isotherms showed that the uptake of water at the interface of [HMIM][Cl] is a reversible process. (paper)