An external airlift loop bioreactor (EALB) was used for production of biomass from natural gas. The effect of riser to downcomer cross sectional area ratio (A(r)/A(d)). volume of gas-liquid separator, superficial gas velocity (U-sgr), and physical properties of gases and their mixtures [u(g) (mu/rho) and D-g] were investigated on mixing time, gas hold-up, and volumetric gas liquid mass transfer coefficients (k(L)a). It was found that A(r)/A(d) has remarkable effects on gas hold-up and k(L)a due to its influence on mixing time. Kinematic viscosity (v(g)) showed its significant role on mixing time, gas hold-up and k(L)a when different gases used (mixing time changes directly whereas gas hold-up and kLa change indirectly). Moreover, it was found that diffusion coefficient of gas in water (D-g) has remarkable effect on k(L)a. The volumetric mass transfer coefficients for methane and its mixtures with oxygen (three different mixtures) were determined at different geometrical and operational factors. In average, the rate of oxygen utilization is approximately 1.8 times higher than that of methane. A gas mixture of 25 vol% methane and 75 vol% oxygen was the best gas mixture for biomass production in the EALB. The correlations developed for predicting the mixing time, gas hold-up, and k(L)a in terms of U-sgr, A(r)/A(d), volume of gas-liquid separator, and gas phase properties have been found to be encouraging. (C) 2009 Elsevier Ltd. All rights reserved.