Understanding the classic precipitation sequence of Al-Cu alloys, solid solution → Guinier-Preston (GP) zones → θ'' → θ' → stable θ, is of academic importance. In situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) techniques were employed simultaneously to study the temperature-dependent behavior of various intermediate precipitation steps in the non-isothermal heating of Al-5.4 wt%Cu alloy. This study quantitatively demonstrates the concurrent evolution of the lattice structure, volume fraction (growth and dissolution) and structural growth in the thickness and length directions with temperature for various intermediate (metastable) precipitates for the first time. The detailed phase transformation mechanism and structural evolution in the precipitation sequence (for GP zones, θ'', θ' and θ phases) can then be resolved. Our data analysis also considered the concurrent existence of multiple precipitates in the precipitation sequence. Moreover, the evolutional behavior of the orientation of precipitates in each precipitation step can be concurrently revealed. Different SAXS analysis models were proposed to successfully interpret the SAXS data. The new information presented by the SAXS/WAXS approach provides insight into the phase transformation mechanism of precipitation in Al-Cu alloys.