The δ13C of fossil leaf cuticle is frequently used for paleoenvironment interpretation. A tacit assumption that is common in such studies is that the δ13C of the cuticle is the same as the δ13C of the original whole leaf. We tested this assumption by measuring the isotopic fractionation between cuticle and whole leaves (ε13Ccuticle-leaf) in 175 phylogenetically diverse species. The average ε13Ccuticle-leaf is indistinguishable from zero (-0.04 ±1.2‰ 1σ), in keeping with the few previously published data and with studies that have tracked the evolution of leaf δ13C during decomposition. Across species, ε13Ccuticle-leaf spans over 9‰: this variability does not covary with growth habit (woody vs. herbaceous) or climate, but does contain a strong phylogenetic signal. In particular, more basal groups (lycopsids and some gymnosperms, basal ferns, and basal angiosperms) tend to have negative ε13Ccuticle-leaf values. This variability should be accounted for in studies that wish to estimate whole-leaf δ13C from cuticle δ13C.