Penning has shown that at 500°C, Cu penetrates into Ni‐doped Ge (containing ∼1016Ni atoms cm−3) to a concentration more than 10 times its thermal equilibrium value. We have confirmed this observation and extended the experiments with the following results. At 500°C the rate of Cu64penetration (in Ni saturated samples) indicates an effective diffusion coefficientDeffof ∼2×10−8cm2/sec and a saturation density,n0, of ∼1016/cm3. No detectable Cu penetration occurs after 10 hrs at 400°C. The Cu penetration at 500°C is independent of dislocation density in the sample and also independent of whether or not the Ni has been previously precipitated. The extra Cu is not electrically active. We have similarly studied the rate of mixing of ordinary Cu and Cu64by diffusion in Ge samples at 500°C and find thatDefffor this process is again ∼10−8cm2/sec. From these experiments we conclude that Cu penetration in Ge is not responsible for the 0.04‐ev energy level which appears during the early stages of Ni precipitation at 400°C, as Penning suggests. The enhanced penetration of Cu64observed at 500°C may be interpreted as resulting from exchange between interstitially diffusing Cu64and precipitates of either Ni or Cu.