Potassium guaiacolsulfonate is an active pharmaceutical ingredient that has been in use for more than a century, and it is still widely used in cough syrups. Nonetheless, no crystal structure has ever been determined to facilitate quality control. This is all the more surprising because two isomers are known to exist. A commercial sample has been studied by X-ray diffraction and thermogravimetric methods. It consists of potassium guaiacol-4-sulfonate, and it crystallizes in the monoclinic space group C2/c as a hemihydrate. The crystals exhibit uniaxial negative thermal expansion, and the electrostatic forces between the ions appear to be the major driving force of the observed displacements. Dehydration of the hemihydrate was observed above 380 K. The anhydrate has a structure similar to the hemihydrate with almost the same unit cell parameters. The dehydration kinetics points to a displacive mechanism; water molecules are located in channels, through which they leave the crystal. The structural readjustments that occur to accommodate the loss of water are driven by electrostatic interactions between the ions similar to the displacements observed for thermal expansion. Under ambient conditions (and humidity), the anhydrate is not stable and transforms into the hemihydrate.