Abstract The phosphorus analogue of piperidine (phosphacyclohexane, II) has been prepared and characterized by NMR. The proton on phosphorus appears as a broad triplet of triplets ( J aa = 12·0 Hz; J ae = 2·5). These couplings require the proton to be axial. No evidence for the presence of an equatorial isomer was obtained. The spectrum of phosphacyclohexane 1-sulfide (III) leads to similar conclusions. For phosphacyclohexane methiodide (IV), however, no conclusions could be reached concerning the location of the proton on phosphorus because of second-order spectral complications. Double-irradiation experiments on IV have showed that 1 J 31PH and 2 J 31PCH 3 , are opposite in sign. The axial conformational preference for the proton on phosphorus in II and III is discussed in relation to other systems.