In this article, water behaviors in solid polymer electrolyte (SPE) membrane that is employed for polymer electrolyte fuel cells (PEFC) are discussed because understanding of water transport in SPE membrane is of great importance in terms of water management in the membrane to maintain performance of PEFC. Firstly, self-diffusion coefficients of water in the SPE membrane were measured using pulsed field gradient spin echo NMR measurement. Two types of SPE membrane. Nafion(R) and Flemion(R), were investigated at various water contents. It is shown that measured self-diffusion coefficients increased with water content in the membrane, although no distinct difference was observed among both membranes. Furthermore, it is demonstrated that the NMR imaging technique can be applied to probe water behaviors in the membrane assembled in fuel cell under operation. It is observed that the membrane gradually dried after the fuel cell began to be operated, resulting in decreasing output current. (C) 2004 Elsevier Ltd. All rights reserved.