Abstract: A simple process for preparing acicular calcium sulphate dihydrate is provided in which a slurry of calcium carbonate containing 100 to 800 grams per liter calcium carbonate is mixed with a solution of sulphuric acid containing 80 to 500 grams per liter sulphuric acid in proportions which produce a calcium sulphate slurry having a pH from 0.5 to 1.5. The process can be used to neutralise by-product sulphuric acid from the preparation of titanium dioxide with crushed limestone. Preferably the calcium sulphate slurry is stirred after formation to reduce the level of unreacted carbonate. The calcium sulphate produced has an aspect ratio of at least 4:1 and has excellent properties for use in paper. The process enables the production of acicular calcium sulphate without employing special crystallisation procedures.