The effective diffusion coefficients of glucose, sucrose and fructo-oligosaccharides in Ca-alginate gel beads at high concentration of sucrose solutions were investigated at $50^{circ}C$. A mathematical model for the kinetics of fructo-oligosaccharide production using immobilized cells was proposed and compared with experimental results varying the bead size, the substrate concentration and the bead ratio. Very low values of diffusion coefficients ranging $1.2-7.6 imes10^{-7} extrm{cm}^2$/sec were obtained, and the predicted results were in good agreement with experimental ones in all cases tested.