The novel approach to interpret the metastable zone width obtained by the polythermal method using the classical theory of three-dimensional nucleation proposed recently [K. Sangwal, Cryst. Growth Des. 9 (2009) 9421 is extended to describe the metastable zone width of solute-solvent systems in the presence of impurities. It is considered that impurity particles present in the solution can change the nucleation rate J by affecting both the kinetic factor A and the term B related with the solute-solvent interfacial energy gamma. An expression relating metastable zone width, as defined by the maximum supercooling Delta T-max of a solution saturated at temperature T-0, with cooling rate R is proposed in the form: (T-0/Delta T-max)(2) = F(1-ZlnR), where F and Z are constants. The above relation can also be applied to describe the experimental data on maximum supercooling Delta T-max obtained at a given constant R as a function of impurity concentration c(i) by the polythermal method and on maximum supersaturation sigma(max) as a function of impurity concentration c(i) by the isothermal method. Experimental data on Delta T-max obtained as a function of cooling rate R for solutions containing various concentrations c(i) of different impurities and as a function of concentration c(i) of impurities at constant R by the polythermal method and on sigma(max) as a function of impurity concentration c(i) by the isothermal method are analyzed satisfactorily using the above approach. The experimental data are also analyzed using the expression of the self-consistent Nyvlt-like approach [K. Sangwal, Cryst. Res. Technol. 44 (2009) 231]: ln(Delta T-maxf/T-0) = Phi+beta ln R, where Phi and beta are constants. It was found that the trends of the dependences of Phi and beta on impurity concentration c(i) are similar to those observed in the trends of the dependences of constants F and Z on c(i) predicted by the approach based on the classical nucleation theory. (C) 2009 Elsevier B.V. All rights reserved.