It is shown that stable carbon isotope ratio measurements in atmospheric volatile organic compounds can be used to determine the photochemical age of the studied compounds. This Isotopic Hydrocarbon Clock has several advantages compared to the conventional Hydrocarbon Clock. Due to the small Kinetic Isotope Effects for the reaction of hydrocarbons with OH-radicals, the photochemical ages derived from changes in isotopic composition are the correct linearly averaged age of the studied compounds, independent of the mixing of air parcels with different photochemical ages. The uncertainties of photochemical ages derived from isotopic composition measurements of atmospheric non-methane hydrocarbons depend primarily on the accuracy of stable carbon isotope ratio measurements, the Kinetic Isotope Effect for the reaction of hydrocarbons with OH-radicals and the variability of isotopic composition of hydrocarbon sources. Uncertainty analysis shows that the Isotopic Hydrocarbon Clock is not only a theoretical possibility but a viable method to determine the photochemical age of atmospheric hydrocarbons. ¿ 2000 American Geophysical Union