Pharmaceutical grade lactose, the purest form of lactose commercially available, is usually contaminated with traces of lactose phosphate (LP), a component of whey, the by-product of cheese manufacturing. This work presents a study of the effects of LP addition on the kinetics of lactose crystal growth during isothermal and cooling seeded-batch crystallisations. The effect of LP is also compared with riboflavin, glucose-6-phosphate and potassium chloride, three other possible contaminants. Two techniques were used to define the growth rate constant (k(g)) of a power law function describing the crystallisation kinetics of ultra-pure lactose. At 30 degrees C, the value of k(g) decreased with increasing LP concentration, following a linear Langmuir adsorption isotherm up to 60 mg LP per kg dissolved lactose (ppm). The particle size distribution was affected for up to 8 h and a proportion of crystals had their shape modified by the presence of LP. Chemical analysis of the final lactose showed that LP was integrated within the crystals. Riboflavin produced changes comparable to LP for growth and size distribution while the other two compounds showed smaller effects. The results for isothermal crystallisations with LP addition were confirmed in the controlled cooling operation. (C) 2006 Elsevier B.V. All rights reserved.