Evolution of microstructure in a binary Al-Cu system (Al-4.3Cu) and a commercially alloyed Al-Cu system (A205) during solution heat treatment was investigated using optical microscopy (OM), scanning electron microscopy (SEM), wavelength-dispersive X-ray spectroscopy (WDS), and differential scanning calorimetry (DSC). The diversified coarseness of the microstructure was initiated by controlling the solidification rate. Different solution treatment temperatures were applied to identify a proper solutioning temperature. The larger microstructural scale required an increased solutioning temperature and prolonged holding time to obtain homogenized solutes in the α-Al matrix. The diffusion of Cu primarily controlled the solution heat treatment process. A diffusion-based model was applied and calibrated to determine the dissolution rate of an Al2Cu particle in the matrix. The model operates on a similar time scale with the experimental results for the Al-4.3Cu and A205 alloys with various microstructural scales, different chemical compositions, and at different solution treatment temperatures. Three-dimensional (3D) reconstructed images from SEM images and energy dispersive spectroscopy (EDS) map of elements showed that TiB2 particles shield the Cu-rich phases in the boundaries of α-Al grains, presumably acting as a physical barrier to the diffusion of Cu solutes toward α-Al grains. The model also suggests that the effective diffusion coefficient of Cu in Al, in the presence of TiB2 particles, reduced by a factor of 2.0–2.5 in the A205 alloy compared with the binary Al-Cu alloy.