Crosslinked poly (styrene sulfonic acid) (CrPSSA) is incorporated into sulfonated poly (ether ether ketone) (SPEEK) through the semi-interpenetrating polymer networks (sIPNs) method to improve proton conductivity of SPEEK proton exchange membranes (PEMs) at low relative humidity (RH). Thermogravimetric analysis indicates that the SPEEK/CrPSSA sIPN membrane can be stable up to 250 degrees C in N-2. A single glass transition temperature (T-g) of the sIPN measured by differential scanning calorimetric measurements suggests good miscibility of the CrPSSA and SPEEK. At fixed RH the proton conductivity of the sIPN membranes increases as more CrPSSA is incorporated into SPEEK membranes. The sIPN membrane with 40 wt.% CrPSSA has a proton conductivity of 10(-3) S cm(-1) at 25% RH, which comparable to that of Nafion115 and is 2 orders of magnitude higher than that of the pristine SPEEK membranes (10(-5) S cm(-1) at RH 50%). The percolation threshold for proton conduction occurs at lower hydrophilic volume fractions with the increasing content of CrPSSA, suggesting different packing behavior of -SO3H groups in the SPEEK/CrPSSA sIPN membranes as compared with SPEEK and Nafion(R) membranes. (C) 2013 Elsevier B.V. All rights reserved.