Precipitation and solid solution hardening have been studied in Al-Cu thin films with Cu content ranging from 0 to 2 wt%. We have changed the precipitate and dislocation structures of films having different Cu concentrations by varying the heat treatments prior to mechanical testing. Pure Al films showed the same values of tensile and compressive yield stresses at a given temperature during stress-temperature cycling. Al-Cu alloy films, however, showed larger stresses in tension than in compression. The compressive flow stress during heating could be changed by a factor of five by the initial heat treatment, but the differences disappeared above 250°C. Upon cooling from 480°C, solution hardening as well as precipitation hardening was observed in the Al-Cu films. Solution hardening is independent of Cu concentration, but for precipitation hardening, both the magnitude and the temperature range in which the mechanism is effective are sensitive to the Cu concentration. The microstructure was observed using transmission electron microscopy. The mechanical behavior is consistent with interactions between dislocations and precipitates which arise due to constraints on the film by the substrate.