Compound specific carbon isotope analyses were performed on a well-characterized suite of unconventional oils and condensates produced from the lower Eagle Ford Formation. Thermal maturity is the major influence on the δ13C values of C3–C19 hydrocarbons. From ∼0.8 to ∼1.1 %Ro, the main stage of oil generation, the δ13C values of most measured hydrocarbons remain relatively constant: n-alkanes are ∼ −30 ‰: light aromatic hydrocarbons, cycloalkanes and isoprenoids are ∼ −29 to −28 ‰. Only the C4 to C10 methylalkanes exhibit high variance with carbon number with δ13C values ranging from ∼ −32 to −28 ‰, respectively. Hydrocarbons from samples ∼1.1 to 1.3 %Ro, the late oil generation stage, are 13C-enriched by ∼1–∼2 ‰. This is the range typically assigned to the influence of thermal maturity on oil from many conventional petroleum systems. The onset of significant oil cracking occurs at ∼1.3 %Ro and all measured hydrocarbons systematically become progressively 13C-enriched although the degree varies by compound class and carbon number. The δ13C values of individual hydrocarbons shift by as much as ∼ +9 ‰ for propane and butane to only ∼ +3 ‰ for methylcyclopentane from <1.1 to 1.7 %Ro. The 13C-enrichments observed in the Eagle Ford oils/condensates with increasing maturity are attributed to the kinetic isotope effect (KIE), whereby 12C–12C bonds cleave faster than 12C–13C bonds. No evidence for isotopic equilibrium was found. The consistency of the sums of the weighted δ13C values for C4–C7n-alkane/methylalkane isomers provides support for Mango's (2000a) theory that these hydrocarbons are generated from precursors with a common chemical structure that pass through a similar transition state.Source facies exert only a minor influence on the carbon isotopic composition of Eagle Ford oils/condensates. Small variances in the 13C values of C6+n-alkanes, cyclohexane and methylcyclohexane are consistent with oils produced east of the San Marco Arch being generated from shales with a higher influx of terrestrial high plant matter compared to the shales west of the Arch. Phase fractionation was shown to have a measurable impact on the δ13C values of the most volatile hydrocarbons in evaporated samples and on the least volatile hydrocarbon that underwent phase separation in the subsurface during production.