A three-dimensional global chemical tracer model of the atmosphere has been adapted to include the stable carbon isotopic composition and isotopic fractionation of ethane and benzene. For computational efficiency the chemistry was based on a prescribed OH-radical concentration field, and therefore the feedback of ethane and benzene chemistry on the atmospheric OH-radical concentrations was not considered. The Emission Database for Global Atmospheric Research (EDGAR) V2.0 emission database used needed to be scaled by a factor of 2.22 in order to have good agreement between observed and modeled concentrations. Modeled isotopic compositions were consistent with the few published observations. The global distribution of modeled stable carbon isotope ratios and the derived mean photochemical ages of ethane and benzene are presented. The model predicts distinct regimes of photochemical aging and air mass mixing for polar regions.