Phosphonic acid polymeric submicrocapsules (PASCs) are synthesized and incorporated into a sulfonated poly(ether ether ketone) (SPEEK) matrix to prepare composite membranes. The microstructure and physicochemical properties of the PASCs and the membranes are characterized by transmission electron microscopy (TEM), energy dispersive X-ray (EDX), field emission scanning electron microscope (FESEM), thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR). Compared with the SPEEK control membrane, the PASC-filled composite membranes exhibit elevated water uptake and proton conductivity at 25 degrees C and 100% relative humidity (RH). The proton conductivity depends strongly on water content within the membranes. Under 40 degrees C and 20% RH, the composite membrane filled with 15 wt.% PASCs (128 nm lumen) shows the highest proton conductivity of 0.0142 S cm(-1) after 90 min testing, about twelve times higher than that of the SPEEK control membrane (0.0011 S cm(-1)), which is positively correlated with the water retention of the membrane. These results suggest that the PASC-filled composite membranes may find encouraging application as efficient water-retention and proton-conduction materials in proton exchange membrane fuel cells (PEMFCs). (C) 2013 Elsevier B.V. All rights reserved.