Three series of nanostructured, μm thick, Co/Ag composite thin films (13, 39 and 61 volume % Co) were produced by dc magnetron sputtering at various substrate temperatures (100–600°C) to determine their magnetic properties and characterize the microstructure. The films were found to be composed of finely dispersed Ag and Co particles. The film surfaces become rougher as the substrate temperature was increased. The crystal lattice structure of the Co was found to be fcc except at the lower substrate temperatures (<300°C) in the 61 volume % films where it was found to be a mixture of fcc and some hcp. The average diffracting particle size of both the Co and Ag rich phases increase with substrate temperature. The magnetic coercivity of the films reached a maximum value when the Co particle size was between 100–150Å.