Presented is an approach to measuring protonic conductivity of powdered samples. The features of this method are the utilization of porous sample holder, which allows for an easy control of the water activity in the sample, the use of spring-loaded contact electrodes, which prevents the loss of contact upon sample shrinking, as well as the implementation of four-electrode ac impedance measurements, which reduces the inaccuracy due to interfacial capacitance of the probes. We also report conductivities of a low equivalent weight version of Nafion in film and powder forms as functions of relative humidity. These data show that the conductivity of a powder is several times lower than the conductivity of a film, which is likely due to the porous nature and intergrain resistances in the powder. (c) 2008 The Electrochemical Society. [DOI: 10.1149/1.2987753] All rights reserved.