We investigated the changes induced in the viscosities of CaO–SiO₂–Al₂O₃–K₂O melts (CaO/SiO₂ molar ratio = 0.68 ± 0.04, Al₂O₃ content = 13.4 ± 0.6 mol%) with the addition of K₂O in amounts of 0–17.4 mol% for temperatures of 1673–1873 K using the rotating crucible method. The viscosity increased with an increase in the K₂O content when K₂O/Al₂O₃ molar ratio < 0.7. On the other hand, the viscosity decreased with the addition of K₂O for K₂O/Al₂O₃ molar ratio > 0.9. The maximum values of the viscosities were noticed in the range of 0.7 < K₂O/Al₂O₃ molar ratio < 0.9. This behavior could not be explained on the basis of the polymerization degree of the aluminosilicate network structure. The ¹⁷O magic angle spinning nuclear magnetic resonance (MAS NMR) spectrum of the 31.6CaO-44.2SiO₂-13.4Al₂O₃-10.8K₂O (mol%) glass suggested that K⁺ ions preferentially compensate the negative charge of AlO₄, while the Ca²⁺ ions primarily create non-bridging oxygens (NBOs). The viscosity data and the ¹⁷O MAS NMR spectrum also suggested that the increase in viscosity for K₂O/Al₂O₃ molar ratio < 0.7 was because of the increase in the average bond strength of the aluminosilicate framework, which was attributable to the substitution of Ca²⁺ ions by K⁺ ions at the charge compensator sites. The activation energies for viscous flow indicated that the NBOs bonded with K⁺ ions, forming NBO-K species, for K₂O contents greater than those corresponding to the viscosity maxima. Thus, the viscosities of aluminosilicate melts are indicative of the average bond strengths of the melts.