The sequence of precipitation in solutionized (SOL) 2124 aluminum and direct-quenched from the die (DQD) 339 aluminum has been identified by a combination of differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). Both alloys form S' (Al2CuMg) as the first precipitate after GP zone dissolution. In each alloy a second phase forms at higher temperatures-Si for DQD 339 Al, theta' (CuAl2) for SOL 2124 Al. These results illustrate two difficulties associated with the interpretation of calorimetric observations. 1) The S' phase precipitates at a much higher temperature in 2124 Al than in 339 Al. Calorimetric determinations of activation energies for GP zone dissolution and S' precipitation suggest that the former is the rate-determining step for the latter. Since this or similar effects can be expected to control precipitation rates in other alloys, a precipitate is not uniquely identified simply by the DSC peak temperature. Accordingly, the literature must be viewed with caution unless the precipitate assigned to a DSC peak is identified by TEM. 2) As Si forms in DQD 339 aluminum, 40% of the S' precipitate dissolves. In this circumstance, where two calorimetrically opposed processes occur simultaneously, activation energies determined by differential isothermal calorimetry are erroneous.