The ternary Al–Co–Si phase diagram was investigated using optical microscopy, powder X-ray diffraction (XRD), differential thermal analysis (DTA) and electron probe microanalysis (EPMA). In the Co-rich part ternary phase equilibria were investigated within two isothermal sections at 1050 and 1200 °C and the homogeneity ranges at these temperatures were determined by means of EPMA. Samples covering the entire ternary system were investigated by means of DTA measurements. A total number of 29 invariant ternary reactions could be derived from these data yielding a complete description of the ternary Al–Co–Si system, including a ternary reaction scheme (Scheil Diagram) and a liquidus surface projection. Diffusion couple experiments were carried out between the end-members Al/CoSi2 at 550 °C, AlCo/Co2Si at 1050 °C and Si/AlCo at 900 °C. The results of these studies are in good agreement with the isothermal sections.