The electrical resistivity of a series of magnesium alloys containing from 300 to 1041 parts per million of gadolinium has been measured from 4.2 °K to room temperature. No low-temperature anomalies have been observed and it is found that Matthiessen's rule is obeyed extremely well over the whole temperature range studied. The electrical resistivity and magnetic susceptibility measurements indicate that the cerium ion in magnesium is in a triply ionized state, and that the contribution to the resistivity can be almost totally accounted for by assuming normal potential impurity scattering. This suggests that the f level of cerium in magnesium lies well below the fermi level of magnesium and that there is very little s–f mixing between the f level of cerium of the conduction electron states of magnesium.