This paper studied the effect of pH and the viscosity of feed solution on the mass transfer in two different specifications of hydrophobic hollow fiber membrane contactors. Experimental results showed that the pH of the feed had a significant effect on the rate of mass transfer, the flux and the removal efficiency of easily ionized volatile compounds (VCs), such as ammonia. In unbuffered feed solutions, both the resistance of membrane and the resistance of feed side were affected by the pH of feed and both of them had contributions to the overall mass transfer. A model, 1/K = {1/K-f + 1/K-m} 1/alpha(NH3)(2) was proposed to predict the effect of pH and gave good agreement with the experimental data for the two contactors. The relationship between viscosities and the mass transfer coefficient could be described by K-1/K-2 =(eta(2)/eta(1))(0.33). Based on our experiments, the change of the viscosities in the concentration range of ammonia from 50 to 10,000 mg l(-1) caused about 3-4% variation of the mass transfer coefficients. Moreover, it may be more suitable to use the equilibrium concentrations of volatile species instead of the total concentrations of VCs to estimate mass transfer coefficients, removal efficiency and flux. Experimental results also revealed that the choice of the pH of feed might be one of the most crucial factors for removing easily ionized VCs, such as ammonia, from water and wastewater. (c) 2004 Elsevier B.V. All rights reserved.