ESR of Nd3+and U3+in CaF2(Bleaney, Llewellyn, and Jones, 1956) provided the first confirmation of the hypothesis of Zintl and Udga˚rd (1939) that charge compensation for a trivalent lanthanum ion is provided by an interstitial fluorine ion. In some cases this is in the nearest interstitial site, as shown by observation of magnetic dipole interaction between it and Ce3+(Baker, Hayes, and O'Brien, 1960). In other cases the charge compensating defect is absent or so far away that cubic symmetry is preserved at the lanthanum site. The effects of x‐ray and ultraviolet irradiation at room and lower temperatures are being studied. Initial results show that Tm3+ions can be converted readily to Tm2+, whose ESR spectrum has been measured (Hayes and Twidell, 1961). This has cubic symmetry, indicating that the Tm3+ion is either accompanied by no defect or by a defect mobile at low temperatures. The spectra of Tm2+and Yb3+are compared. The work of Sierro (1961) on the hydrolysis of CaF2is discussed.