Metastable CuCo solid solutions are produced by vapour condensation onto a substrate held at 5 K. Co solved in Cu causes a negative temperature coefficient of the electrical resistivity at low temperature (resistance anomaly). The resistance anomaly depends on the square of the Co concentration up to about 3 at% Co. This result agrees with the observation that single Co atoms have no localized magnetic moment in a non‐magnetic host but get a moment by interaction. Annealing the CuCo films to e.g. Ta = 390 K leads to clustering of the Co atoms. With the Co atoms in magnetic clusters, the resistance anomaly varies linearly with Co concentration. The residual resistivity as a function of Co concentration follows straight lines always and the slope of these lines decreases with annealing.