Gas transfer from bubbles to the dense phase in a fluidized bed is described as a combination of diffusional and convective processes. In the convective process the cloud formation is important as is the adsorption of the gas at the solid interface. The resulting differential equations are worked out numerically and for many cases the theoretical transfer coefficients are given. Numerical solutions were also obtained in cases when a first order chemical reaction occurred at the solid interface. A comparison with existing theories is made. In a following contribution experimental values of the transfer coefficient will be given as measured in two- and three-dimensional beds.