Abstract The 34 S isotope effect on the chemical shift of P III nuclei in P (SR) 3 esters [R= Et,(C 12 H 25) n, Ph] is close to 20.6 ppb per P III [BOND] 34 S bond, ie 3–4 times larger than for thiolo sulphur atoms in S= P (SR) 3 or O= P (SR) 3. A similar behaviour is noted for the P ([BOND] S [BOND]) 3 sites in P 4 S 9 and P 4 S 3 and for the downfield triplet of P 4 S 8, which, accordingly, is assigned to the tri-coordinate P atoms of this sulphide. 13 C isotope ...