The electrical resistivity of irons containing different levels of impurity have been measured at 77° and 293°K for a range of grain sizes. The resistivity of a given iron is found to vary with the grain diameter (dmm) as &rgr; =A+Blog(1/d).AandBare both temperature dependent, increasing with increasing temperature. With increasing impurity content, the parameterAincreases andBdecreases. Thus the purer iron shows a larger variation of resistivity with grain size than the less pure material. The observed relationship can only be explained in terms of a grain‐boundary scattering process if the effective width of the boundaries is much greater than is usually believed.