The phase diagram of the Al-Cu-Si ternary system was investigated experimentally at the temperatures 600 degrees C and 800 degrees C. The current study was designed to contribute to a better understanding of the ternary phase diagram Al-Cu-Si at elevated temperatures, where some of the phase equilibria were only tentatively known. It was found that the ternary phase tau proposed earlier is actually an extended solid solution of Al in the CuSi_delta phase. At 700 degrees C, the phase CuSi_delta is stable as a pseudo ternary phase, and the existence of a significantly extended thermal stability range in the ternary was also confirmed by DSC measurements. In agreement with previous isothermal sections at other temperatures, a huge solubility of aluminum in CuSi_kappa and negligible solubility of Si in AlCu_beta was observed.