The microstructures and mechanical properties of hot and cold sputtered Al-1 wt.%Si and Al-1 wt.%Si-0.5 wt.%Cu films were studied. Transmission and scanning electron microscopies were used to examine the grain size, precipitate morphology and surface structure of the films. Stress measurements using a substrate curvature technique and hardness measurements made by nanoindentation were employed to study the mechanical properties. The cold sputtered films show a smaller grain size than the hot sputtered films before and after annealing, whereas the precipitate morphology looked the same after annealing for both films. Stress/temperature curves of AlSi and AlSiCu films are very similar on heating and on cooling above 220 degrees C. However, below this temperature the tensile stresses in the AlSiCu films increase sharply with decreasing temperature compared with the AlSi films. This effect is normally attributed to precipitation hardening of Al2Cu precipitates which form below 250 degrees C. However, there is evidence for a second explanation. The steep rise in stress can be caused by the end of diffusional relaxation. Calculations show that the breakdown of thermal activation of diffusional processes occurs in the same temperature range.