Interdiffusion coefficient, \ ildeD, in Cu-Mn alloys has been determined by Matano’s method in the temperature range between 748 and 930°C for the various couples consisting of pure copper and the Cu-Mn alloy. \ ildeD increases with manganese content up to 30 at%Mn and decreases remarkably with further increase of manganese. Concentration dependence of the activation energy and the frequency factor for interdiffusion show a small peak at 20 at%Mn and a very large peak at 80 at%Mn; the former is attributed to the increase of rigidity of the alloy and the latter to the deviation from the ideal solution at respective concentration of manganese. It has been found that the Kirkendall marker moves toward the manganese-rich side and the Kirkendall voids form on the manganese side, showing that the manganese atom diffuses faster than the copper atom. From the marker shift, the intrinsic diffusion coefficients, D_Mn and D_Cu, in 29 and 77 at%Mn alloys have been determined, and the results show that the ratio of D_Mn⁄D_Cu is 1.5∼2.9.