Two new colour centres in alcaline earth fluorides are investigated in this article. Both are due to the impurities oxygen and yttrium and they are only observed with EPR after the crystals have been X rayed. In fact the X rays change the valence state of the impurity complexes formed during growth or by hydrolysing the appropriately doped crystals thus creating the paramagnetic centres. One of them consists of a (YO₂) molecule substitutionally located in the crystal. The other centre involves one 0^- ion in substitutional position nearby a Y³⁺ ion. Their model and the informations about their electronic structure have been deduced from EPR, ENDOR, optical measurements, annealing experiments and in the case of the (YO₂) also by investigating the centre produced with water enriched in ¹⁷O. The results are described by the appropriate spin Hamiltonians. The centre (YO₂) is shown to form approximately a (Y³⁺ — O₂^3-) structure. The other centre has its magnetic electron located predominantly in a p_z orbital of the O^- ion involved. The thermal annealing experiments performed on samples containing both centres and Y³⁺ ions exhibit after appropriate treatment of the samples charge transfert from the less stable centre involving one O^-ion to the (YO₂)^- molecule ion (transfert of holes). The annealing experiments further show that the optical transition observed at 486 mμ in CaF₂ and at 510 mμ in SrF₂ arises from the (YO₂) centre.