C19H19NO3·x CH3CN (x=0.3),Mr=643.35, hexagonal, space groupP61 (No. 169),a=23.027(5),c=5.775(1) A,V=2652(1) A3,Z=6. The structure was solved by direct methods and refined toR=0.077 for 1562 observed MoKα reflections. The title heterocyclic carboxylic acid was established as thetrans isomer, with the phenyl and carboxyl substituents occupying pseudo-equatorial and equatorial positions, respectively, of the piperidin-2-one ring in a half-chair conformation. Acid host molecules related by the 61 screw operation are linked by intermolecular O−H...O (cyclic amide) hydrogen bonds to generate an open channel bounded by coaxial intertwined helices each having a pitch of 5c. Within each channel of free diameterca. 6.0 A the acetonitrile molecules partially occupy highly disordered sites which do not lie on thec axis.