The crystalline field theory of paramagnetic salts has been applied by Jordahl (1934) to the determination of the magnetic susceptibilities of certain cupric salts, and from a comparison with the existing experimental data it has been concluded that the Cu ++ ion in these salts can be regarded as acted on by a predominantly cubic crystalline electric field due to the surrounding ions of the crystal. The cupric salts considered by Jordahl were cupric sulphate pentahydrate, CuSO 4 .5H 2 O, and the potassium and ammonium double sulphates, CuSO 4 .K 2 SO 4 .6H 2 O and CuSO 4 .(NH 4 ) 2 SO 4 .6H 2 O respectively. In the case of the first of these, X-ray investigations (Beevers and Lipson 1934) have shown that the Cu ++ ion is surrounded by an approximately regular octahedron of six negatively charged oxygen atoms, which gives rise to an electric field of nearly cubic symmetry. In the case of the double sulphates no structure determinations have been made but it is again probable that the Cu ++ ion is acted on by a cubic field arising from a regular octahedral arrangement of the six water molecules around it. de Haas and Gorter (1930) have measured the mass susceptibility of powdered cupric sulphate pentahydrate down to a temperature of 14° K, and by suitably choosing the parameters of the crystalline field Jordahl was able to account approximately for the variation of susceptibility with temperature. There are, however, systematic differences between the calculated and observed curves which become pronounced at low temperatures. The cupric double sulphates have been investigated in powdered form down to a temperature of 80° K by Janes (1935), who also concluded that the experimental results could be explained on the basis of a predominantly cubic electric field acting on the Cu ++ ion. The present paper is mainly concerned with the temperature range below 20° K, and the absolute values are given for the susceptibilities of the three cupric salts in powdered form down to a temperature of 1.6° K. In the case of each of the double sulphates the mean susceptibility is found approximately to follow a Weiss law with a small negative value of θ from 290 down to 14° K, but in the helium range θ becomes zero within the limits of experimental error and a simple Curie law is then obeyed down to the lowest temperature. The mean susceptibility of the cupric sulphate can be represented over the whole temperature range by a Weiss law with a negative θ rather larger than that found for the double sulphates.