We develop a detailed framework for calculating uptake rates of gases on fractal particles. Aggregates with a fractal dimension between 1.6 and 2.6 are generated numerically. Three-dimensional simulations of uptake on these aggregates are performed for a large range of conditions. The numerical results can be expressed by a single physically based formula as a function of the usual parameters (gas phase diffusion coefficient, uptake coefficient) and of morphological parameters (aggregate gyration radius, size of the monomers composing the aggregate, fractal dimension). The expression for the uptake flux on an aggregate is generalized to a lognormal distribution of aggregates. The results for an isolated aggregate are compared to calculations used for spherical particles. The importance of accounting for the fractal character of aggregates in the calculation of the uptake flux is highlighted. The magnitude of the errors also depends on which aggregate equivalent size is used as the characteristic radius for mass transfer. ¿ 2000 American Geophysical Union