The laser ultrasonic monitoring of aluminium alloy microstructural development during thermal processing is demonstrated. This monitoring is based on the ability to measure the differential ultrasonic attenuation in alloy systems. Differential attenuation is related to anelastic physical processes which are active during microstructural evolution. For the aluminium alloys studied, these anelastic effects derive from alloying species solution and mobility. Methods have been outlined for separating processes which affect the physical properties but are not related to microstructural changes from those properties which are linked to microstructural evolution.