The influence of SiC particulate size on the age-hardening response of 2009 aluminium has been monitored utilizing hardness, electrical conductivity and differential scanning calorimetry. Ageing involved either two or three stages depending on the reinforcement size. For 2009 Al reinforced with 29 mu m SiCp, initial ageing consisted of GPB (Guinier Preston Baqaryatsky) I zone formation. Decreasing the particulate size to 4 mu m eliminated GPB I formation. This suppression of GPB I formation during ageing suggests that decreasing SiC size decreases the vacancy supersaturation following quenching by providing additional vacancy sinks at SiC/matrix interfaces. Subsequent ageing involved, at the larger reinforcement sizes, a transition with increasing time from GPB I zones to GPB Il-dislocation complexes. At smaller (4 mu m) reinforcement sizes, the formation of GPB Il-dislocation complexes occurs directly. Finally, the last stages of age hardening in all SiCp-reinforced composites examined consisted of heterogeneous nucleation of S’/S and GPB II --> S’/S transformation.