Crystallite formation in membranes cast from sulfonated poly(etheretherketone) (SPEEK) dispersions in dimethylacetamide has been investigated by differential scanning calorimetry. The crystallites melt at 220 degrees C and the melting enthalpy (Delta H-m) is strongly influenced by environmental conditions of solvent evaporation. When solvent is evaporated in air, Delta H-m decreases with increasing the evaporation temperature so that no crystallites are formed above 45 degrees C. The study of crystallite formation at 30 degrees C, under controlled relative humidity (RH) and solvent evaporation rate, has shown that Delta H-m decreases i) with decreasing RH from 75% to 5%, at constant evaporation rate, and ii) with increasing evaporation rate, especially at low RH. The exchange of protons with sodium ions results in an increase of the crystallite melting temperature thus indicating the presence of sulfonic groups in the crystallites. In comparison with crystallite-free SPEEK membranes, the presence of crystallites turns out in enhanced membrane mechanical properties, reduced hydration in liquid water above 75 degrees C, strongly anisotropic swelling (occurring mainly in the through-plane direction) and better performance in H-2/air fuel cell at 70 degrees C. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.