In this study the solution annealed properties (T4) and solution annealed and aged properties (T6) of nominal 1/4-inch sections (6 mm) of die cast AZ91D and AM60B were studied, including the effects of aging time and temperature, and grainsize. An annealing time of 20 hours was found to be optimum for aging response and strength. Both the as-cast and heat treated properties of die cast specimens were superior to those of gravity-cast-and-heat-treated specimens. The T6 treatment did not improve the properties of die cast parts with nominal 6 mm sections due to grain growth induced by the T4 anneal. The effect of long time exposure to 250° F (120 °C) on the mechanical properties of as-cast die cast AZ91D and AM60B castings was also studied. It aged them to higher strengths than attainable by T6 treatments. Maximum strengthening occurred in 30-60 days; after 90 days overaging occurred.