The composite membranes were prepared by dispersing particles (ZrP) or nanosheets (ZrP-NS) of layered zirconium phosphate hydrate (Zr(HPO4)(2)center dot nH(2)O) in sulfonated poly(etheretherketone) (SPEEK) with various ZrP or ZrP-NS contents (0-50 vol.%). Their chemical stability and proton conductivities at 40-150 degrees C were investigated, and compared for ZrP- and ZrP-NS-composite membranes. The enhanced chemical stability was confirmed in both composite membranes. The results of thermogravimetry (TG)-differential thermal analysis (DTA) and diffuse reflectance Fourier transform infra-red (FT-IR) spectroscopy suggests that hydrogen-bonds were formed at both the SPEEK/ZrP and the SPEEK/ZrP-NS interface and that enhanced stability is attributed to these bonds. The membrane with 50 vol.% ZrP-NS showed a high conductivity of 7.9 x 10(-2) S cm(-1) at 150 degrees C under saturated water vapor pressure, and a smaller relative humidity dependence of conductivity at 130 degrees C than the SPEEK membrane with 50 vol.% ZrP. The SPEEK/ZrP-NS composites were shown to have promising potential as membranes usable at 150 C. (C) 2010 Elsevier B.V. All rights reserved.