The growth rates of the (010) face of alpha-lactose monohydrate crystals were measured at 30, 40 and 50 degrees C in the relative supersaturation range 0.55-2.33 in aqueous solutions. The mechanisms of growth were investigated. Spiral growth was found to be the mechanism of growth up to a critical relative supersaturation (s-1)(crit)=1.9 at 30 degrees C. Above the critical relative supersaturation, the crystal growth mechanisms were predicted to change. All growth models fit equally well to the growth rates. No two-dimensional nucleation was observed above critical supersaturation by AFM. On the other hand increased step height and roughness on the edges of steps were observed. It was concluded that the growth mechanism of the (010) face of alpha-lactose monohydrate crystal is spiral growth. A parabolic relationship was obtained below critical supersaturation followed by a linear relationship with relative supersaturation. Crown Copyright (c) 2009 Published by Elsevier B.V. All rights reserved.