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The crystal structure of the title compound (common name HEPES), C8H18N2O4S, has been redetermined at 100 K in order to properly elucidate the protonation state of the HEPES molecule. The piperazine ring has a chair conformation and one of the N atoms in the ring is protonated, which was not previously reported [Gao, Yin, Yang, & Xue (2004). Acta Cryst. E60, o1328–o1329]. The change of protonation state of the nitrogen atom significantly affects the intermolecular interactions in the HEPES crystal. The structure is stabilized by N—H...O and O—H...O hydrogen bonds and ionic interactions, as the title compound in solid state is a zwitterion. HEPES molecules pack in layers that are held together by ionic and weak interactions, while a hydrogen-bonded network connects the layers.