Abstract13C chemical shifts and31P13C spin–spin coupling constants are reported for 10 alkyl‐, 20 benzyl‐ and 3 (naphthylmethyl)‐phosphonates. While in saturated aliphatic chains P–C couplings over more than four bonds cannot be resolved, couplings over up to seven bonds are observed in the benzyl type systems. Conformational and substituent effects onJ(PC) are studied and discussed.nJ(PF) (n = 4, 5, 6) are reported for the isomeric (fluorobenzyl)phosphonates andnJ(PP) (n = 5, 6, 7) were obtained from the13C satellites in the31P n.m.r. spectra of the isomeric diphosphonates, C6H4[CH2P(O)(OEt)2]2. Comparison of those13C absorptions of the latter, which represent the X parts of ABX or AA′X spin systems, with the spectra of the corresponding (methylbenzyl)phosphonates, CH3C6H4CH2P(O)(OEt)2, yielded the relative signs ofnJ(