N-Unsubstituted, N-methyl and N-benzyl cis- and trans-2-(hydroxymethyl)cyclohexylamines were subjected to ring closure with phenylphosphonic dichloride, phenyl dichlorophosphate and bis(2-chloroethyl)phosphoramidic dichloride in order to synthesize P-epimeric diastereomers of the corresponding unsubsituted and N-substituted 2-phenyl-, 2-phenoxy- and 2-[bis(2-chloroethyl)amino]octahydro-2H-3,1,2-benzoxazaphosphinine 2-oxides. The stereochemistry and conformations of the prepared compounds were analyzed mainly by variable-temperature1H, 13C and 31P NMR spectroscopy. Geometry optimizations were performed for some trans-fused molecules by utilizing the B3LYP DFT method and a locally dense basis set, followed by JH,P coupling constant calculations at the UB3LYP/cc-pVTZ level of theory. The population of the N-out conformation of the cis-fused compounds was found to increase with increasing steric size of the N substituent (H < Me < CH2Ph), and also to be strongly influenced by the configuration at the phosphorus atom. The heteroring of the trans-fused compounds predominantly adopts a chair conformation with the notable exception of the (2R*,4aS*,8aS*) epimers of the 2-phenoxy-substituted compounds, for which the axial tendency of OPh causes the heteroring to adopt mainly B2,4a- and/or 1S4a-type nonchair conformations. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)