The ionic conductivity of Nafion® 1100 extruded membranes re-cast from solutions of butan-1-ol and propan-2-ol is measured in 0.5 mol dm–3 H2SO4 at 295 K, using an immersed, four-electrode d.c. technique. The general trend is an increasing conductivity for the thicker membranes. Materials which were solution-cast from butan-1-ol yielded the highest conductivity while a series of membranes with lower conductivities (similar to those of an extruded Nafion® 1100 series of membranes) was found using propan-2-ol. The conductivity results indicate that membranes manufactured by extrusion and casting from various solvents might have different structures. Differences in the water content and conductivity of the membranes are considered to arise from the impact of processing conditions on the surface and bulk structure of the membranes.