The polymorphism of Ba(OH)(2) described in the literature, has been investigated by crystallographic, infrared spectroscopic, and calorimetric techniques. From the X-ray powder diffraction pattern it has been found that the beta phase has a monoclinic structure (space groups P2(1) or P2(1)/m) with a = 6.7835(6), b = 7.9268(6), c = 9.4246(10) Angstrom and beta = 95.823(9)degrees. A phase transformation to alpha-Ba(OH)(2) occurs at 526 +/- 1 K, which upon cooling in a dry atmosphere remains unchanged. However, a reversible phase transition alpha --> beta is observed in the presence of traces of water vapour, acting as a catalyst. The thermodynamic properties of alpha-Ba(OH)(2), including fusion, have been measured. In addition, the fusion properties of Sr(OH)(2) are discussed.