Part I of this study described a calorimetric investigation of kinetics and energetics of Guinier–Preston (GP) zone formation and precipitation in solutionized (SOL) aluminum alloy 2124. The present paper discusses results of experiments designed to determine the effect of prior GP zone formation on precipitation in the same alloy. The samples studied (designated SOL/GP samples) had each been held at temperatures ≤100°C until GP zone formation was determined to be complete, after which differential scanning calorimetry (DSC) and differential isothermal calorimetry (DIC) precipitation measurements were carried out. The DSC data (for scan rates from 1.2 to 20°C/min) were analyzed by the Kissinger method to yield activation energies and time constants. From the DIC experiments, we obtained precipitation kinetics information using analysis techniques developed in this laboratory. Precipitation activation energies and time constants for SOL/GP samples derived from DSC agree rather well with those from DIC. As was the case for the SOL alloy, two processes with slightly different temperature ranges are involved in precipitation in SOL/GP 2124. Although prior GP zone formation seems to have little effect on the first precipitation process (associated with S′ (CuMgAl 2)), it does reduce the activation energy of the second one (assigned to θ′ (CuMg 2)) by more than 10%. The exothermic heat of precipitation, Δ Q (measured isothermally) is also relatively unaffected by previous GP zone formation.