The use of flocculent microorganisms in fermentations is a very promising application. However, it has some drawbacks of which the most important ones are related to mass transfer limitations within flocs. Their mechanical fragility and their shape present extra difficulties when evaluating parameters such as effective diffusivity (De) and external mass transfer coefficient (Kc). This work focuses on the determination of De and Kc for glucose and oxygen transport in aggregates of Saccharomyces cerevisiae. A modified diffusion cell was used in order to avoid the destruction of the flocs and an image analysis technique was applied for floc size determination. De and Kc were calculated using two methods: an analytical one, based on the solutions of Fick's law of diffusion and a numerical one, based on general mass balances of a component in flocs and the bulk phase. Diffusion coefficients were found to be 1.10 × …; s… for glucose and between 0.049 × … and 0.21 × … for oxygen. These values are around 17% and 0.2 to 1% of the respective diffusivities in pure water. They were not affected by the initial solute's concentration.