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The adsorption phenomena of sodium pyrophosphate and sodium silicate were studied on synthesized diamond and unreacted graphite. Both depressants adsorbed on diamond surface, while they were rarely onto unreacted graphite. It suggests that the diamond surface adsorbed with depressant becomes hydrophilic to depress the floatation of diamond. The adsorption isotherm curves were also measured to estimate the differential heat of adsorption. They were between the typical values corresponding to physical and chemical adsorption.The dependency of adsorption density for these depressants on pH showed as follow: primary adsorption species was HP2O73-and adsorption isotherm equation was of Langmuir type for sodium pyrophosphate, while it was SiO (OH) 3-and Freundlich type for sodium silicate.