Banana–plantain fibers (BPFs) were treated with acetic anhydride (AA), epichlorohydrin (EP), and an acetic anhydride–epichlorohydrin blend (AA_EP). Natural fiber–polyester composites (PCs) were fabricated using untreated and treated BPF by the hand lay‐up technique. Modified BPF Raman and Infrared spectroscopy (ATR‐FTIR) analysis revealed losses of lignin and hemicelluloses and new chemical bonds that confirmed the replacement of part of the raw fiber hydroxyl groups, which gave rise to a degree of hydrophobicity in the treated fibers. This change altered their water sorption isotherms, swelling behavior in water, impedance spectroscopy, and XRD analysis. Assays of water absorption and tensile resistance for untreated BPF (U) and modified BPF polyester composites demonstrated a remarkable reduction in water intake, whereas tensile strength was maintained ( P < 0.05) for the modified BPF polyester composites. POLYM. COMPOS., 37:2960–2973, 2016. © 2015 Society of Plastics Engineers