Graphene oxide (GO) and its phosphonated analogue (pGO) have been incorporated into sulfonated poly(styrene-isobutylene-styrene) (SO3H SIBS) to generate membranes with enhanced water retention. The polymer nanocomposite membranes (PNMs) were prepared per SIBS sulfonation level (i.e., 38, 61, and 90 mole %), filler type (i.e., GO and pGO) and filler loading (i.e., 0.1, 0.5 and 1.0 wt.%). FT-IR and TGA confirmed the functionalization and incorporation of the fillers into SO3H SIBS. No significant changes were observed in the thermal stability or FTIR spectra of the PNMs after addition of the fillers. Dissimilar behaviors were observed for the water absorption capabilities (i.e., swelling ratio and water uptake) after incorporation of the fillers. The nanofillers enhanced the water absorption of the sulfonated polymer, possibly due to interconnections between the ionic groups. Therefore, the PNMs could not only potentially function as proton exchange membranes (PEMs) for several applications such as direct methanol fuel cells (DMFCs).