We have discovered that excess configurational entropy plays an important role in the thermodynamic K-Na mixing behavior of alkali feldspars. We initiated this project by increasing the precision of our earlier-measured enthalpies of mixing (H_ex) via HF solution calorimetric measurements on large numbers of K-Na exchanged samples for 3 new feldspar series: (1) a 21-member low albite – microcline series, (2) a 20-member analbite – sanidine series, and (3) a 14-member low sanidine series. Combined with earlier results for adularia- and orthoclase-based series (Hovis, 1988, J. Petrology), the new data not only increased precision relative to earlier work, but quantitatively confirmed the systematic decrease in H_ex magnitude with increased Al-Si disorder. The new H_ex data then were combined with Gibbs free energies of mixing (G_ex) based on earlier-reported reversed alkali-exchange equilibria between molten (K,Na)Br salts and low albite – microcline series members (Delbove, 1975, Amer. Mineral.) as well as analbite – sanidine and adularia series members (Hovis, Delbove & Roll, 1991, Amer. Mineral.), which produced expressions for the total entropies of mixing (S_ex,total) for these series. Heat-capacity data and the associated excess vibrational entropies (S_ex,vib) for low albite – microcline (Benisek, Dachs & Kroll, 2014, Amer. Mineral.) and analbite – sanidine (Haselton, Hovis, Hemingway & Robie, 1983, Amer. Mineral.) provided the means for separating excess configurational entropy (S_ex,conf) from S_ex,vib, where S_ex,conf = S_ex,total – S_ex,vib. The calculations produced strongly negative values of S_ex,conf for disordered alkali feldspars, but strongly positive ones for both of the ordered series. Although short-range K-Na order can explain negative S_ex,conf values, there is no apparent explanation for seemingly impossible positive S_ex,conf. In assessing this result, note that the precision of H_ex and G_ex are high due to the large numbers of samples utilized in the solution calorimetric studies to determine H_ex and the tightly reversed alkali exchange equilibria in the determination of G_ex. Moreover, there is sample consistency in the data, as feldspars utilized for all studies except that of Delbove (1975), including the Cp measurements, came from this laboratory utilizing the same synthesis techniques.