Concentration of nn-dipoles, nnn-dipoles and charge carriers as well as parameters related to their reorientation and mobility have been estimated from an analysis of a.c. conductivity data of polycrystalline samples of the Sr1-xThxF2+2x solid solution. The molar fraction of charge carriers has been evaluated as a function of composition. Results obtained confirm, in agreement with the clustering process model, that the (F-i)(m) ions responsible for long range motions in Sr1-xThxF2+2x are interstitial fluoride ions of F triple prime type.