The effect of the addition of 5, 10, and 20 wt% of the alkali oxides on the viscosity of a haplogranitic melt composition has been investigated at 1 atm and in the temperature range of 400–1650 °C. The high-temperature viscosity data were obtained with concentric cylinder viscometry and the low-temperature viscosity data using micropenetration vis-cometry. The combined data sets for low- and high-temperature viscosities have been fitted for each composition using the Tamann-Vogel-Fulcher (TVF) equation. The effect of alkali oxide on the viscosity of a haplogranite melt is extreme. The viscosity decreases with added alkali oxide content in a nonlinear fashion. The first few mole percent of alkali oxide added decreases viscosity several orders of magnitude, whereas subsequent addition of alkali oxide has a much smaller effect. The effects of each of the alkalis are broadly similar, implying that the structural role of the alkalis is common to all. In detail however, the viscosity of the strongly peralkaline melts investigated here increases with the size of the added alkali cation in the order Li <Na <K,Rb,Cs. This trend probably reflects a minor influence of the alkali-O bond strengths on the melt viscosity. This distinction of a dominant depolymerizing influence and a minor alkali specific bond-strength influence has important implications for the comparison of these data with those for the addition of other depolymerizing agents on the viscosity of haplogranitic melt (e.g., H2O, F2O−1).