In most polysaccharide fermentations, the nature of the fermentation broth changes drastically with time and, as a result, the overall oxygen mass transfer coefficient (K(L)a) can vary by orders of magnitude. To obtain a better understanding of this phenomenon, an experimental program was devised to study the respective influence of molecular weight and concentration of dextran solutions on K(L)a. Experiments were conducted in a reciprocating plate bioreactor. This bioreactor uses a stack of perforated plates that is reciprocated axially in the column and it is therefore well suited for mixing viscous liquid broths and providing uniform overall mass transfer coefficients. The variation of K(L)a with the power input per unit volume and the superficial gas velocity were obtained for three ranges of molecular weights and five concentrations of dextran. In every medium, two regimes of operation were observed as a function of the power input per unit volume : a first regime, at low power inputs per unit volume where K(L)a remains constant until a threshold of power input is attained; and a second regime, which is characterized by a steep increase of K(L)a as a function of the power input per unit volume. The presence of dissolved biological macromolecules, not only because of their effect on the rheology of the medium but also because their effect on the gas-liquid interface, has a significant impact on K(L)a. It was found that, generally, small concentrations of polysaccharide favor oxygen mass transfer despite the increase in medium viscosity. However, the respective influence of polysaccharide concentration and molecular weight was different for the two regimes of operation.