Al–0 to 8.1 mass (0 to 8.9 at)% Mg solid solution alloys were annealed at 623 K for 3.6 ks and then water quenched. Resistivity at 77 and 300 K (ρ77 and ρ300) was measured. According to a corrected Nordheim's law, the relations between resistivity and atomic fraction of solute Mg, X, are obtained as follows;ρ300/nΩm=27.66+550.6X(1−1.72X)ρ77/nΩm=2.319+527.5X(1−1.72X)The factor before the X (1 − 1.72 X) term becomes larger at elevated temperature of resistivity measurement. This fact shows that the solute Mg atom in aluminum has a positive deviation from Matthiessen's Rule, giving η=Δρ300/Δρ77= 1.044. The best fit relation between resistivity ratio R= ρ300/ρ77 and ρ77 is ρ77/nΩm=25.26/(R−1.043) +0.0019. Using most reliable relation between temperature and equilibrium solid solubility of magnesium to aluminum, the temperature-concentration-resistivity chart has been obtained. Utilisation of the chart is introduced.