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 (ϵ¹³C_cuticle-leaf) in 175 phylogenetically diverse species. The average ϵ¹³C_cuticle-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, ϵ¹³C_cuticle-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 ϵ¹³C_cuticle-leaf values. This variability should be accounted for in studies that wish to estimate whole-leaf δ¹³C from cuticle δ¹³C.