The corrosion protection afforded by cerium, lanthanum and praseodymium conversion coatings formed on pure magnesium and a magnesium alloy, WE43, has been studied using dc polarisation and ac impedance techniques. The coatings, which were formed by immersion in rare-earth salt containing solutions, reduced significantly the dissolution of magnesium in a pH 8.5 buffer solution. With continued immersion of the treated electrodes in the aggressive pH 8.5 solution, the coatings first appeared to become more protective, but after periods exceeding 60 min began to deteriorate. This is attributed to the formation of magnesium hydroxy corrosion products and mixed rare earth/magnesium oxide/hydroxide coatings, which on continued immersion became consumed by the formation of magnesium corrosion products.