The habit modification of crystals of fine chemicals by impurities can cause poor filtration or centrifugation, with serious consequences on manufacturing rate. A method for identifying those impurities, which act as habit modifiers in crystallisation from solution, is described, and its use in an industrial case exemplified. The procedure involves identifying those impurities which form solid solutions in the product crystals, and testing these individually and in combination for their habit-modifying effect in an otherwise pure system.