An X-ray study of 5-methyl-1-thia-5-azacyclo-octane 1-oxide perchlorate reveals that the molecules undergo a change in orientation in the unit cell when the temperature is lowered from 25 to 3°. The form stable at 3°, or α-form, has cell dimensions a 9·87, b 8·78, c 13·26 Å, and β 97°54′; the space group is P2₁/c. The form stable at 25°, or β-form, has cell dimensions a 20·10, b 8·89, c 6·77 Å, and β 97°48′; the space group is P2₁/a. In both forms, there are four molecules of C₇H₁₆ClNO₅S(F_w 261·5) in the unit cell. Full three-dimensional analyses have been carried out on each form and it is found that the conformation of the eight-membered ring is an approximate ‘boat-chair’ and is effectively unchanged by the transformation. Another feature of the structure is the existence of a transannular N ⋯ H ⋯ OS hydrogen bridge with a N ⋯ O separation of 2·65 ± 0·01 Å.