A predictive model of carbon isotope fractionation (ε(p)) and abundance (σ13C(phyto)) is presented under circumstances where photosynthesis is strictly based on CO2(aq) that passively diffuses into marine phytoplankton cells. Similar to other recent models, the one presented here is based on a formulation where the expression of intracellular enzymatic isotope fractionation relative to that imposed by CO2(aq) transport is scaled by the ratio of intracellular to external [CO2(aq)], c(i)/c(e). Unlike previous models, an explicit calculation of c(i) is made that is dependent on c(e) as well as cell radius, cell growth rate, cell membrane permeability to CO2(aq), temperature, and, to a limited extent, pH and salinity. This allows direct scaling of c(i)/c(e) to each of these factors, and thus a direct prediction of ε(p) and δ13C(phyto) responses to changes in each of these variables. These responses are described, and, where possible, compared to recent experimental and previous modeling results.