In this present paper, the effect of different contaminants (single and mixture) on the variation of alpha-factor (a ratio of processing water to clean water volumetric mass transfer coefficients, or k(LaProcess water)/k(LaClean water)) was investigated. Tap water and aqueous solutions with surfactants, NaCl, glucose and inert microorganism were chosen as liquid phases and an elastic membrane with a single orifice as gas sparger. This study has clearly shown that the presence of these contaminants affects the bubble generation phenomenon, the interfacial area (a) and the different mass transfer parameters, such as the volumetric mass transfer coefficient (k(L)a) and the liquid-side mass transfer coefficient (k(L)). The contamination of surfactants, even in small quantities, has shown the most significant effects. The determination of the alpha-factor in terms of interfacial area (alpha(a)) and k(L) coefficient (k(L)a) was applied to understand details of the differences of the overall alpha-factor. The liquid surface tension and the alpha-factor as alpha(a) and alpha(kL) prove to be crucial for predicting the changes of alpha-factor in the mixture of various contaminants. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.