The precipitation in two solution-treated Al-Cu-Si alloys, one with and one without free silicon particles, was studied by differential scanning calorimetry, X-ray diffraction and hardness measurements. In both alloys, theta’-phase and Si-phase precipitation processes proceed simultaneously. As in binary Al-Si and Al-Cu alloys aged at the same temperature, precipitation occurs at clearly separate time intervals; this finding suggests an interaction between the two precipitation processes. It is shown that the interaction of stress fields around newly formed precipitates can result in a significant decrease of the nucleation barrier and thus can explain the synchronous precipitation. The activation energy of the combined precipitation reaction is 0.95 +/- 0.06 eV.