The bulk impedance parameters of polycrystalline samples of the Sr1-xBixF2+x (0 less than or equal to x less than or similar to 0.50) solid solution with fluorite-type structure were determined from an analysis of ac conductivity data, using a modified Almond-West model. Hopping rates of mobile F- ions were extracted from frequency dependences of conductivity at different temperatures. The fraction of charge carriers (per unit cell) in the samples investigated has been estimated. The nature of charge carriers has been identified on the basis of the clustering process model and neutron diffraction data : the interstitial ions of F’ type appear as responsible for long range motions in Sr1-xBixF2+x.