Fouling generally occurs above the so-called critical flux, below which steady-state membrane permeability is assumed to be attainable. Operation at sub-critical fluxes can thus be used to minimize membrane fouling. However, rejection behavior may be affected as a consequence of operating within this sub-critical mode that sustains the desirable permeate flux. In this study, the effluent from a synthetic activated sludge production process was used in the assessment of the performance of membrane microfiltration, as a pretreatment in desalination for wastewater reuse. The critical flux was identified using the step-by-step technique. Different operating regimes i.e. above and below the critical flux were used to assess the relationship between solute rejection and membrane fouling. When operating at sub critical mode, rejection was constant even under increasing transmembrane pressure (TMP). This arises mainly from the back transport of particles in the absence of cake formation. Beyond the critical regime, cake formation occurred and rejection increased with increasing TMP. At the critical regime, a decline in rejection was obtained. This rejection behavior was consistent over the three pore sizes that were investigated. Increasing the pore size appears to decrease the rejection at both regimes. This is because larger pore size allows the transmission of smaller particles and a less compact cake formation under and above the critical flux regime respectively. It appears from this study that one may be able to use rejection behavior to confirm and determine the critical flux and adds to the confidence of using the step-by-step method to determine the critical flux.