The effect of Ca/Mg-resinate concentration on solution viscosity was studied in toluene. The viscosity was observed to increase drastically above a specific concentration value. The change in viscosity dependence on concentration implies a transition from a dilute to a concentrated regime, where viscous dissipation is governed by resinate molecular interactions and excluded volume effects. The concept of critical resinate concent ration, c(crit), was applied and used to explain the nonlinear solution-viscoity increase during the resinate synthesis. Ca/Mg-resinates were synthesized by the fusion method at 265 degrees C in a laboratory-scale batch reactor. During the syntheses the increase in 50 wt % toluene solution viscosity of the resinate turned almost exponential at a specific reaction time, denoted by t(50). Based on the apparent reaction kinetics, a semiempirical model with two estimated parameters was derived for the solution viscosity to be used for the prediction of the t(50) time of resinate syntheses.