Method for determining acid gas storage capacities of an underground geologic medium by means of a multiphase reactive transport model.The geologic medium is divided into a set of sub-volumes for which connections are defined. A multiphase reactive transport model is then defined by dividing, according to a transfer rate principle, the phenomena involved in the behaviour of said geologic medium into two parts. A first part is treated by a first sub-model of “compositional multiphase reservoir” type allowing the transfer rates linked with the gas phase to be determined. A second part is treated by a model of “single-phase reactive transport” type allowing the transfer rates linked with the water and the solid to be determined. The model is initialized by means of data acquired in the field or in the laboratory. The acid gas storage capacities of the medium are determined from the reactive transport model.Application: acid gas storage in a natural reservoir.