The interaction of H2O vapor with flame soot has been investigated in the molecular flow regime with use of the molecular diffusion tube technique over a sizable temperature range. The primary real-time data consist of time-dependent mass spectrometric signals and enable the determination of the initial uptake coefficient γ0MC, the surface residence time τs of adsorbed H2O, and the number ns of adsorption sites per square centimeter of soot substrate surface after applying a Monte Carlo trajectory model that accounts for surface saturation by the H2O pulse propagating across the tube. Typical values at 298 ± 2 K are γ0MC < 2 × 10-3 and τs < 5 ms for toluene, acetylene, and diesel soot whereas decane soot does not show any measurable interaction at 298 K. A detailed study of the interaction of H2O with well-characterized decane soot at lower temperature results in the following Arrhenius parameters for desorption of H2O from gray soot generated from a fuel-rich diffusion flame, log(1/τs) = (8.8 ± 0.5) − (7.0 ± 0.5)/RT, and from black soot generated in a lean decane diffusion flame, log(1/τs) = (8.5 ± 0.5) − (9.0 ± 0.6)/RT with R = 1.987 × 10−3 kcal/(mol K). These expressions reveal τs of 160 ms at 193 K and 400 ms at 243 K for gray and black decane soot, respectively. Spiking the fuel with thiophene (C4H4S) up to 500 ppm mass fraction (0.05%) does not lead to any change in the H2O adsorption behavior, and saturation experiments with H2O pulses reveal the limited number of H2O adsorption sites on soot accounting for a few percent of the surface carbon atoms. Some atmospheric implications are discussed.