Features of reactions between Cu and Al in the solid state are studied by example of Cu–20% wt. Al system depending on the mixing conditions of the components: either mechanical alloying (MA) of elemental powders followed by their sintering; thermal diffusion interaction between layers in multilayered Cu/Al foils deposited by electron-beam physical vapour deposition (EBPVD). Phase transformations induced by the thermomechanical treatment of both the powders and multilayered foils (MFs) are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). As shown, the MA during 8 hours results in the formation of disordered $\beta$-phase (structural type A2) with a body centred cubic lattice (b.c.c.). Heating of the powders up to 500°C followed by their sintering at 850°C and 30 MPa leads to ordering of $\beta$-phase into $\gamma_2$-Al$_4$Cu$_9$ phase (type D8$_3$). In case of the MFs, heating at 150°C results in the formation of two-phase structure: $\beta$ + $\theta$-Al$_2$Cu. The formation of monoclinic $\eta_2$-AlCu phase and its growth are observed as temperature increases up to 200–400°C, while phase composition of the multilayered foils at 500–850°C corresponds to $\eta_2$-AlCu + $\gamma_2$-Al$_4$Cu$_9$. The difference of the phase transformations in systems prepared using mechanical alloying (MA) and MFs is caused by different transformation mechanisms: diffusion in powders and shear in MFs. The effect of change of the phase composition on mechanical properties of Cu–20% wt. Al composites are considered.