A new method of determining experimentally the magnetic susceptibilities of gases and vapours is described. Compensation is effected for the test bulbs, and the influence of surface condensation, if any, of gases and vapours can be allowed for and separately investigated. Electromagnetic retorsion is employed and the arrangement is rendered independent of small changes in the magnetizing current. The molar susceptibility of carbon dioxide was found to be -(20.79 ± 0.08) × 10 -6 , showing good coincidence with the values of other investigators. The arrangement is being improved in several directions to give much greater accuracy. A systematic investigation of many gases and vapours will then be undertaken.