Results of an experimental study of the effect of sample volume on the metastable zone width of pure unstirred potassium nitrate aqueous solutions investigated as functions of cooling rate R and saturation temperature To of the solution by the polythermal method are described and discussed. The metastable zone width was measured as the maximum undercooling Delta T-max during the cooling of 4 and 100 mL solutions at constant predefined rates between 5 and 40 by registering changes in the intensity of transmitted light using the UV-vis spectrophotometer and observation of visible nuclei by the visual method, respectively. The experimental data of the maximum undercooling Delta T-max corresponding to saturation temperature To for the two investigated volumes as a function of cooling rate R were analyzed by employing self-consistent Wyklike approach and an approach based on the classical three-dimensional nucleation theory. The results suggest that the dependence of the value of the metastable zone width of unstirred solutions on their volume is associated with the diffusion processes in the solution. (C) 2013 Elsevier B.V. All rights reserved.