The environmental transport and fate of nanomaterials are largely dependent on their colloidal stability. To date, the stability of reduced graphene oxide (RGO) materials is not well understood. We examined the electrokinetic properties and aggregation kinetics of three RGOs synthesized by reducing graphene oxide (GO) with N₂H₄, NaBH₄, and L-ascorbic acid. In NaCl solution, the critical coagulation concentrations of the materials correlated reasonably with their C/O ratios. However, the increased aggregation tendency of RGOs was caused mainly by their increased hydrophobicity rather than decreased surface charge negativity. For both monovalent and divalent cations, less densely hydrated cations were more effective in causing aggregation than more densely hydrated cations (e.g., K⁺vs. Na⁺, and Ca²⁺vs. Mg²⁺) owing to the greater efficiency of the former in neutralizing surface charges and the bridging effect in the case of Ca²⁺. The RGOs exhibited high stability in artificial surface water, whereas different degrees of aggregation were observed in artificial groundwater. Strikingly, in artificial groundwater the aggregation tendency of the materials correlated poorly with the degree of reduction but strongly with the types and concentrations of surface O functionalities, which determined the nature and strength of interactions between RGO and cations in solution. The findings further underline that the stability of nanomaterials is controlled by the complex interplay between nanomaterial surface properties and solution chemistry factors.