This investigation was concerned with the effects of high temperature treatment in vacuo or in rarefied oxygen atmospheres upon the effective electrical resistivity of dilute (copper‐rich) CuMn, CuFe, and CuCo rods and wires. The CuMn alloys studied most extensively contained between 1 and 12 at. % manganese while the CuFe and CuCo alloys contained between 0.1 and 0.3 at. % iron or cobalt. It was found that the solutes in the CuFe and CuCo wires and rods become rapidly oxidized (internally) during heat treatment for several hours at 990°C in pure oxygen pressures of 0.5 μ or greater, while in 0.05 μ O2 this oxidation is absent and previously oxidized specimens are reduced. These observations are based on measurements of electrical resistivity, and in the case of CuFe, substantiated by measurements on the magnetization. Heat treatment of these CuFe and CuCo wires and rods near 1000°C for several hours either in vacuo (0.1 μ air) or 5 μ O2 does not appreciably affect the cross‐sectional uniformity of these specimens. In the case of CuFe alloys, the magnetic behavior demonstrates the virtually complete oxidation of solute to Fe3O4 after prolonged anneal in an oxidizing atmosphere. The behavior of the CuMn alloys was found to be more complicated, partly because of the high solute concentrations involved. It was found that wires and rods of CuMn rapidly become nonuniform in solute concentration throughout their cross section when annealed near 1000°C for several hours either in a vacuum (0.1 μ air) or in 5 μ of pure oxygen; this nonuniformity of manganese concentration develops at a considerably higher rate in the 5 μ O2 atmosphere than in vacuum, and leads to erroneous values of the resistivity and to erroneous curves of resistivity vs temperature for CuMn wires or rods. On the other hand, annealing of such wires for several hours in vacuum at temperatures around 800°C does not appreciably disturb the cross‐sectional uniformity. Finally, it is emphasized that the ``carrier gas'' technique (involving a flow system with small concentrations of oxygen mixed with helium at atmospheric pressure, for example) is entirely unsatisfactory for experiments on internal oxidation, particularly if one wants to know the oxygen partial pressure.