Six scale inhibitors were evaluated to reduce the scaling tendency of an effluent water produced from the western zone of the Kuwait Oil Fields, commingled with three different formation waters. The effluent water to formation water ratios for each of the water combinations under study were chosen to reflect worst case conditions, as predicted from a computer compatibility study. In this study, static autoclave jar tests were conducted prior to dynamic scaling tests in order to select an appropriate dose concentration for the inhibitors. All scale inhibitors were tested under reservoir conditions at a fixed dose of 15 ppm. The results of the static autoclave jar test and the dynamic scaling test indicated that the most effective scale inhibitor was an aqueous solution of organic phosphates at an optimum dose rates of 15 and 20 ppm, respectively for CaCO3 and BaSO4 scales. Keywords: scaling, autoclave, jar test, dynamic scale testing, effluent water, formation waters.