To characterize the oxygen mass transfer in a fermentation system and to study the efficiency of mixing devices, model fluids are often used so that experimental conditions can be better controlled. In this study, wood pulp suspensions were used in an attempt to mimic the rheological properties of fermentation broths of Aspergillus niger. Two different types of bioreador were used: a reciprocating plate bioreactor and a stirred (Rushton) bioreactor. The oxygen mass transfer coefficient (K(L)alpha) was measured for various mixing intensities, airflow rates and wood pulp concentrations, and a correlation of K(L)alpha as a function of the power input per unit volume and the superficial gas velocity was derived for each bioreactor and each pulp concentration. K(L)alpha was found to increase with agitation and air flow rate, and was adversely affected by an increase in pulp concentration in the case of the reciprocating plate bioreactor.