The high-pressure behaviour of Si0.8Ge0.2 alloy is explored using in situ Raman spectroscopy, X-ray diffraction techniques and density functional theory (DFT) simulations. High pressure experiments revealed a pressure-induced transition from the stable cubic semiconducting phase (dc- Si0.8Ge0.2) to the tetragonal β-tin metallic phase (β-Si0.8Ge0.2) during compression. This sluggish transition is significantly accelerated at moderate temperature (< 300 °C). Upon decompression, successive transitions towards metastable phases are observed. A first transition from the metallic β-Si0.8Ge0.2 toward the rhombohedral r8-Si0.8Ge0.2 phase is observed at 10.3 GPa followed by a partial transition to the body-centered cubic bc8-Si0.8Ge0.2 phase at 2.2 GPa. After releasing the pressure, r8 and bc8 phases coexist at ambient conditions. This transition pathway is similar to that followed by pure silicon and is consistent with the ab initio enthalpy calculations. This phase transition sequence is confirmed by in situ Raman spectroscopy, where signatures of r8 and bc8 phases are observed in the Raman spectra at decompression. An ab initio simulation method is proposed to assign the Raman spectrum of Si0.8Ge0.2 alloy using group theory and projection operators. The exploration of metastable states in these alloys is of major interest both in terms of applications (e.g. optoelectronics) and from a fundamental point of view to better understand the effects of alloying on the physical properties (e.g. vibrational).