The crystal structures of diphenyl (cycloheptylamido)phosphate, C 19 H 24 NO 3 P or (C 6 H 5 O) 2 P(O)(NHC 7 H 13 ), ( I ), and diphenyl (dibenzylamido)phosphate, C 26 H 24 NO 3 P or (C 6 H 5 O) 2 P(O)[N(CH 2 C 6 H 5 ) 2 ], ( II ), are reported. The NHC 7 H 13 group in ( I ) provides two significant hydrogen-donor sites in N—H...O and C—H...O hydrogen bonds, needed for a one-dimensional hydrogen-bond pattern along [100] in the crystal, while ( II ), with a (C 6 H 5 CH 2 ) 2 N moiety, lacks these hydrogen bonds, but its three-dimensional supramolecular structure is mediated by C—H...π interactions. The conformational behaviour of the phenyl rings in ( I ), ( II ) and analogous structures from the Cambridge Structural Database (CSD) were studied in terms of flexibility, volume of the other group attached to phosphorus and packing forces. From this study, synclinal (± sc ), anticlinal (± ac ) and antiperiplanar (± ap ) conformations were found to occur. In the structure of ( II ), there is an intramolecular C ortho —H...O interaction that imposes a + sc conformation for the phenyl ring involved. For the structures from the CSD, the + sc and ± ap conformations appear to be mainly imposed by similar C ortho —H...O intramolecular interactions. The large contribution of the C...H/H...C contacts (32.3%) in the two-dimensional fingerprint plots of ( II ) is a result of the C—H...π interactions. The differential scanning calorimetry (DSC) analyses exhibit peak temperatures ( T m ) at 109 and 81 °C for ( I ) and ( II ), respectively, which agree with the strengths of the intermolecular contacts and the melting points.