C10HI4NOPS, Mr= 227-27, orthorhombic, P2~2~2~ at 105K, a= 11.654 (3), b= 13.301 (4), c=7.193(3)/~, V=lll5.0(ll)A 3, Z=4, Dm(293 K)= 1.26 (1), Dx(105 K)= 1.354 (1) gcm -3, 2(Mo Ka) = 0.71073/~, /t(Mo K~t) = 3.88 cm -~, F(000) = 480, T= 105 (1) K, R = 0.032 for 2645 observed reflections. On crystallization the compound spontaneously resolves into enantiomers. The absolute configuration found (R,R) confirms previous interpretations of NMR data. The hetero- cyclic five-membered ring is only slightly puckered. The P-S and P--N bond lengths are 1.9382(5) and 1.6536 (14)/k respectively. Comparisons with similar structures show that these exocyclic bond lengths correlate inversely with the endocyclic X-P-Y angle. Introduction. The stereochemistry of substitution reac- tions at trivalent phosphorus in a series of cyclic compounds has been investigated by Nielsen & Dahl (1984). From the magnitude of certain NMR coupling constants an assignment of the configuration around P was made. For the compounds (1) the diastereomers with small NMR coupling constants 3Jpocu were assigned a structure with the substituent on P trans to the methyl group of the five-membered ring.