This study aimed at investigating whether stable isotopes can be used to monitor the progress of volatile organic compounds (VOCs) volatilization from contaminated sediment during venting. Batches of a dry aquifer sediment were packed into stainless steel HPLC columns, humidified with distilled water and later contaminated by either liquid toluene or propan-2-ol. The VOCs were then volatilized by a stream of gas at room temperature, and the concentrations and stable isotope ratios of gaseous VOCs were recorded by isotope-ratio mass spectrometry. During early stages of volatilization of toluene, the isotope ratios Δδ13C shifted to more negative values by about −3 to −5‰ and the Δδ2H by more than −40‰, while the concentration remained at or near initial saturated vapor concentration. Depletion of the isotope ratios in the gas was explained by the vapor-liquid fractionation process, which is amplified by successive self-partitioning steps of gaseous VOC into remaining liquid VOC. For propan-2-ol the carbon isotope shift was negative like for toluene, whereas the H shift was positive. Hydrogen bonding in the liquid propan-2-ol phase causes a normal vapor-liquid H isotope effect which was described already in classical literature. The isotope shifts in the present experiments are larger than previously reported shifts due to phase-change processes and reach the magnitude of shifts usually observed in kinetic isotope fractionation.