The shear viscosities of Cl-bearing melts in the system Na2O-Fe2O3-Al2O3-SiO2 were determined at temperatures of 550–950 °C in the range of 108.5 to 1012.0 Pa s using the micropenetration technique. The compositions are based on addition of Fe2O3 or FeCl3 to aluminosilicate glasses with a fixed amount of SiO2 (67 mol%). Although there was loss of Cl− during the glass syntheses, no loss occurred during the viscometry experiments. It is to be expected that Cl− takes the structural position of O2−, and thus reduces the polymerization of the melt structure, and therefore the viscosity of the melt; as F− does. Our measurements show that the presence of Cl− increases or decreases the viscosity of the melts as a function of melt composition. In the present melts, at least 10% of the Fe exists as network-modifying or charge-balancing Fe2+; whereas the rest exists as network-forming Fe3+. It is proposed here that the different effects of Cl on viscosity are due to the preferred Cl−-Fe2+NBO bonding together with the different structure of peralkaline and peraluminous melts. In peralkaline aluminosilicate melts, the addition of Cl2O−1 will destroy 2 NBOs and create one BO if Cl− bonds primarily to the Fe2+ creating non-bridging O atoms. This would result in an increase in viscosity. In peraluminous melts, the addition of Cl2O−1 may result in Cl− bonds to the charge-balancing Fe2+, creating 2 new tri-clusters [assuming (Al3+,Fe3+)Si2O5 tri-clusters exist]. The preference of Cl− to form bonds to the NBO-forming Fe2+ is indicated by the small amount of Cl− soluble in the peraluminous melt structure in comparison to that soluble in the peralkaline structure.