AuCo solid solutions have been prepared by co-evaporation of both components onto a liquid-helium-cooled substrate. The electrical resistivity of these solid solutions was then measured in situ after different annealing treatments (Ta=80K, 150K, 290K, 360K) as a function of temperature and Co concentration (c=0.25-23 at.%). The residual resistivity increases linearly with Co concentration up to c=8 at.%. The negative slope of the resistivity d rho /dT taken at 4K is proportional to the square of the Co concentration (up to c=1.8 at.%). The c2 law, valid for the random solid solution (Ta=80K), means that the temperature-dependent cross section for the electrons is due to interactions between the Co atoms. Annealing the films to Ta=360K allows clustering and deviation from the random distribution of the Co atoms in Au. For this case the negative slope d rho /sup //dT is proportional to c3/2 in agreement with the bulk sample results of Loram et al. (1970).