This paper describes the use of the drainage gradient method for assessing the relative inhibitory effectiveness of various reverse osmosis (RO) anti-scalants. Five different anti-scalants were selected for the examination of the drainage gradient method's feasibility for evaluating their relative inhibitory effectiveness. The results show that pH, Delta K-Ca (increase in cycles of concentration) and Langelier's saturation index (LSI) can be used as indicators and that Delta K-Ca( is the most suitable indicator. The sensitivities of the three indicators from greatest to least are: Delta K-Ca > LSI >> pH, with relative standard deviations (RSD) which are 38.39%, 14.22%, and 1.45%, respectively. The order of the scale suppression effectiveness of the five tested anti-scalants, based on the three judgment indicators, is D>E>B>C> A. (C) 2014 Elsevier B.V. All rights reserved.