Abstract: Heating at 100℃ for more than 30 min caused a marked increase in viscosity of barium sulfate suspensions (100% w/v) containing 0.5% sodium laminarin sulfate (formula No. 1) or 0.6% sodium hexametaphosphate (formula No. 2). The rheological property of these suspensions were also changed from a Newtonian flow of the formula No. 1 and dilatant flow of the formula No. 2 to a pseudoplastic flow with thixotropic character.Quantitative determination of sulfate ion formed from heated solution samples showed that the rate of increase in concentration was parallel to the rate of increase in viscosity. The same studies on hexametaphosphate containing suspensions gave similar results except that its decomposition rate was lower.Sodium phosphate and sodium sulfate were experimentally shown to flocculate barium sulfate suspension upon incorporation. It is concluded that the formation of phosphate and sulfate ions by thermodegradation of laminarin sulfate and hexametaphosphate was the cause for increase in viscosity. Suspensions (100% w/v) containing 3% sodium citrate (formula No. 3) exhibited a dilatant flow with thixotropic property and was more viscous than that containing other deflocculating agents. It was noted that sodium citrate containing suspension was more thermostable. The original viscosity (40 cp) did not change much after heating at 100℃ for 120 minutes (52 cp).