Knowledge of the glass transition temperatures (T(g)s) as function of composition reflects miscibility (or lack of it) and is decisive for virtually all properties of polymer-based materials. In this article, we analyze single blend-average and effective Tgs of miscible polymer blends in full concentration ranges. Shortcomings of the extant equations are discussed to support the need for an alternative. Focusing on the deviation from a linear relationship, defined as Delta T-g = T-g - phi T-1(g,1) - phi T-2(g,2) (where phi(i) and T-g,T-i are, respectively, the weight fraction and the T-g of the i-th component), a recently proposed equation for the blend Tg as a function of composition is tested extensively. This equation is simple; a quadratic polynomial centered around 2 phi(1) - 1 = 0 is defined to represent deviations from linearity, and up to three parameters are used. The number of parameters needed to describe the experimental data, along with their magnitude and sign, provide a measure of the system complexity. For most binary polymer systems tested, the results obtained with the new equation are better than those attained from existing T-g equations. The key parameter of the equation a(0) is related to parameters commonly used to represent intersegmental interactions and miscibility in binary polymer blends. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 80-95, 2008