Abstract Potassium hexatitanate, K 2Ti 6O 13, m.p. 1370 ± 15°C congruent, is obtained from the K 2OTiO 2 system in supercritical water or molten KCl in the form of colourless fibrous crystals 0·1–5 μ, averaging about 1 μ, in cross sectional dimensions and up to more than 1 cm in length. Crystals of similar fibrous form obtained from K 2OTiO 2KF melt systems in molten KCl appear to be a thermally and hydrolytically unstable potassium oxyfluotetratitanate isostructural with potassium tetratitanate, K 2Ti 4O 9. The fibrous potassium oxyfluotetratitanate is converted by water extraction followed by heat treatment to potassium hexatitanate without any change of crystal form. Syntheses, structural characteristics, and physical and chemical properties of new fibrous potassium titanates are described.