The colloidal stability of functionalized graphene sheets (FGSs) in aqueous sodium dodecyl sulfate (SDS) solutions of different concentrations was studied by optical microscopy and ultraviolet visible light absorption after first dispersing the FGSs ultrasonically. In up to similar to 10 mu M SDS solutions, FGSs reaggregated within a few minutes, forming ramified structures in the absence of SDS and increasingly compact structures as the amount of SDS increased. Above similar to 10 mu M, the rate of reaggregation decreased with increasing SDS concentration; above similar to 40 mu M, the suspensions were colloidally stable for over a year. The concentration of similar to 40 mu M SDS lies 2 orders of magnitude below the critical surface aggregation concentration of similar to 1.8 mM SDS on FGSs but above the concentration (similar to 18 mu M) at which SDS begins to form a monolayer on FGSs. Neither surface micelle nor dense monolayer coverage is therefore required to obtain stable aqueous FGS dispersions. We support our experimental results by calculating the van der Waals and electrostatic interaction energies between FGSs as a function of SDS concentration and show that the experimentally observed transition from an unstable to a stable dispersion correlates with a transition from negative to positive interaction energies between FGSs in the aggregated state. Furthermore, our calculations support experimental evidence that aggregates tend to develop a compact structure over time.