The thermodynamic properties and phase diagram of the system Bi-Zn were optimized using Lukas' program BINGSS. Special efforts (including the pre-optimization treatment of data) were employed to reproduce the known flat, asymmetric, and convex shape of the miscibility gap. A comparison of calculated values with their experimental counterparts is given. The basic statistical characteristics of the solution obtained are presented. Previous assessments of the system are analyzed and compared with the present endeavor.