The microstructure of Co–Cu-based (Co0.75Si0.10B0.15)100−xCux alloys was investigated for a wide range of Co/Cu ratios, with a focus on liquid phase separation and amorphous phase formation. The simultaneous occurrence of liquid phase separation and amorphous phase formation was confirmed in rapidly solidified (Co0.75Si0.10B0.15)100−xCux (x = 10–50) alloys. Nano-scale emulsion-like structures were found in the macroscopically phase-separated Co-rich region in the (Co0.75Si0.10B0.15)100−xCux (x = 30, 50) alloys. The microstructure in the Co–Cu–Si–B alloys strongly depended on the solidification process and the Co/Cu ratio.