In this paper, the various concepts of idealmixtures found in the open literature are analyzed and compared. It is in particular shown that expressions conventionally admitted for the ideal-mixture propertychanges on mixing are incorrect when the idealmixture is obtained from pure components which are not in the same aggregation state as the mixture. Among these properties, a special attention is devoted to the ideal-mixture Gibbs energy change on mixing since it is used to determine the equilibrium conditions. After explaining how to properly estimate this quantity, it is shown how to correctly perform the double-tangent construction of coexisting phases for binary systems modeled with the gamma-phi approach. (C) 2012 Elsevier Ltd. All rights reserved.