About 3000 experimental points obtained in 220 miscellaneous experiments published in 57 papers have been processed simultaneously in order to obtain the most reliable Gibbs energy of theYBa2Cu3O6+zsolid solution in the temperature range from 250 to 1300 K. A part of this solution is well-known as the “Hi-TcY123” phase. All other thermodynamic properties of the solution including the conditions for the tetragonal-orthorhombic phase transition and the miscibility gap at lower temperatures, are derived from the assessed Gibbs energy. The linear error model introduced recently by one of the authors has been employed for the simultaneous assessment. The results obtained are compared with those of the conventional weighted least squares method and the benefit of the new approach is discussed. Another problem in simultaneous assessment that is also considered is visualizing the quality of the fit. New types of graphs (partly based on the linear error model) that facilitate visualizing the quality of the fit are presented. (A critical review with 72 references.) ©1998 American Institute of Physics and American Chemical Society.