Three coagulants, polyaluminum chloride (PACl), aluminum sulfate (Alum) and cationic polyacrylamide (cPAM) are used in this investigation. A nephelometric turbidity monitoring system (NTMS), assembled from a nephelometric turbidimeter and a data acquisition unit, is used continuously to monitor online the turbidity data in a jar test. Both calculated standard deviation (SD) of the measured turbidity data and results of an analysis of a digital image are used to discuss the effect of coagulants on the floc strength. The experimental results reveal that the diameter of flocs is strongly correlated with the SD values calculated from the collected turbidity data during the coagulation process. Also, the floc strength factor that is calculated from the turbidity SD is linearly related to the theoretical strength (sigma) that is obtained by analysis of a digital image. The strength factor, theoretical strength (sigma), floc strength coefficient (C) and stable size exponent (gamma) all follow the order cPAM>PACl>Alum in this study of kaolin coagulation. Additionally, the recovery factor that was determined from turbidity SD values is highest for the cPAM floc. Therefore, this study reveals that the NTMS provides a new and easy way to compare the floc strengths of flocs that form from different coagulants or different coagulation mechanisms, and can eliminate the need for complex image analyses. (C) 2011 Elsevier B.V. All rights reserved.