The work is devoted to the viscosity determination of molten slags from the joint smelting of oxidized nickel and sulfide copper ores. The model slags were synthesized close in composition and structure to real samples, and limiting the investigated composition range (wt.%): iron-containing slag (8.9 CaO, 11.8 MgO, 12.5 Al₂O₃, 47.4 SiO₂, 13.3 FeO, and 5.0 Fe₂O₃) and iron-free slag formed by removing iron oxides from the iron-containing slag (12.5 CaO, 16.0 MgO, 9.4 Al₂O₃, and 58.3 SiO₂). The experimental (vibrational method) viscosity values for iron-containing (1550-1300 °C) and iron-free (1550-1400 °C) model slags are 0.31-2.33 and 1.28-4.55 Pa·s, respectively. Viscosity estimation by the Kalmanovitch–Frank model shows a discrepancy with the experimental data for iron-containing slag. Regression analysis (by Weimann–Frenkel–Urbain formalism) of the experimental data was carried out, and original empirical models were proposed to predict the primary model slags viscosity in the considered temperature range. Reducing the basicity from 0.7 (iron-containing slag) to 0.6 (iron-free slag) increases viscous flow activation energy from 204 to 236 kJ/mol. To maintain optimal slag viscosity, joint smelting of oxidized nickel and sulfide copper ores should be carried out at a temperature not lower than 1400 C.