The effects of sulfates on the adsorption and dispersing properties of cement paste of polycarboxylate (PCE) superplasticizer were studied. The effects of sulfates on the hydration process of cement paste were systematically investigated through isothermal calorimetry as well as scanning electron microscope (SEM). The results show that the addition of sulfates reduces the adsorption of PCE on cement particles and hence impairs the dispersing capability of PCE. Among the sulfates, alkali sulfates (Na2SO4 and K2SO4) most significantly influence the dispersing capability of PCE, while the poorly soluble CaSO4?2H2O slightly influences the dispersing capability of PCE. A small amount of sulfates retards induction period, promotes the maximum hydration rate, and increases the heat evolution rate due to the transformation from ettringite (AFt) to mono calcium aluminate (AFm). A high amount of sulfates significantly advances the maximum hydration rate peak at acceleration period, promotes the formation of AFt, and impairs the formation of hydrated calcium aluminate (CAH). Among the sulfates, the addition of MgSO4 greatly retards the hydration heat peak at acceleration period and significantly promotes the transformation from AFt to AFm. After cement paste is hardened, a large number of filamentous hydrated sulphoaluminate phases are present due to the addition of MgSO4, because MgSO4 greatly retards the hydration process and slows the nucleation and precipitation of hydration products.