AbstractCarbon‐13 NMR spectra of 56 long‐chain polybranched carboxylic acids their esters, including chloroand acetoxy‐substituted derivatives, have been studied. An empirical linear two‐parameter equation, relating carbon chemical shifts in α‐branched carboxylic acids and their derivatives to the chemical shifts of the corresponding carbon atoms in analogous hydrocarbons, and to the degree of branching at the α‐carbon atom,\documentclass{article}\pagestyle{empty}\begin{document}$$ {\delta_{\rm x} = {\rm A}\delta_{\rm H}+{B_n}+{C}} $$\end{document}is suggested for the assignment of the signals, where δXis the shift in the acid (ester), δHthe shift in its hydrocarbon counterpart andnthe degree of branching. The regression coefficients,A,BandC, are given for X=COOH, COOMe and COOEt.It is shown that with polysubstituted derivatives, the equation may be applied sequentially. The secondary coefficients for the α‐substituent then depend on the nature of the substituent X entering the primary equation. The equation may, if necessary, be applied in combination with the additive and structural increment