The crystal structures of the two title piperidine derivatives show different conformations for the six-membered heterocycle. TheN-substituted 4-piperidinone 1-[(1R)-2-hydroxy-1-phenylethyl]piperidin-4-one, C13H17NO2, (I), has a chair conformation, while the piperidine substituted in position 2 with a thiocarbonyl group, 8-[(1S)-1-phenylethyl]-1,4-dioxa-8-azaspiro[4.5]decane-7-thione, C15H19NO2S, (II), features a half-chair conformation. Comparison of the two structures, and data retrieved from the literature, suggests that the conformational flexibility is mainly related to the hybridization state of the C atom α to the piperidinic N atom: a Csp3atom favours the chair conformer, while a Csp2atom distorts the ring towards a half-chair conformer. In the crystal structure of (I), weak C—H...O hydrogen bonds link the molecules into supramolecular chains propagating along theb-axis direction. In the crystal of (II), the molecules are linked by weak C—H...S contacts into supramolecular chains propagating along theb-axis direction.