The structure of S,8-methano- r-4-phenyl c4a, c5,6,7 c8 c8ahexahydro4 H 1,3 -benzoxazin- 2 (3 H)-thione (C 15H 17N0S, M r=259.37) has been established by X-ray crystallography from diffractometer data: it crystallizes in the monoclinic space group P2 1/n with a=6.150(2) Å, b=9.655(1) Å, c=22.093(4) Å,β=96.75(2)† V=1302.7(8) Å 3,4,D c=1.32gcm -3and p( Cu K) =20.4cm -. The structure has been solved by direct methods, refined to R=0.050 for 2193 observed reflections. The X-ray analysis substantiated the structure: the NMR spectra in- dicated that the 4-phenyl group assumes an exo-equatorial position. The puckering parameters of D. Cremer, J.A. Pople, J. Am. Chem. Soc., 97 (1975), 1354 (ref.1), of the distorted hetero ring (a transitional form between E 4-envelope, ( 5S 4-screw-boat) show that, depending on the positions of the hetero atoms, both the norbornane, norbornene skeletons markedly alter the characteristic transitional ( 5E/ 5H 6) shape of the 1,3-oxazine ring observed in other saturated, partly saturated l,3-benzoxazin-2-ones, analogous thiones.