A process and an apparatus for treating exhaust gas from an internal combustion engine include at least two exhaust-gas treatment modules. An exhaust gas stream can be at least partly deflected depending on a load state of the internal combustion engine in such a way that at least parts of the exhaust gas flow through one or more modules. This makes it possible to advantageously construct and operate even an exhaust gas system of large-volume internal combustion engines, in which conversion and treatment of the exhaust gas is carried out in individual modules, even in no-load operation, basically at very low exhaust gas mass flow rates. The individual modules can be adapted to various load levels of the internal combustion engine. A rail-borne vehicle and a water-borne vehicle having the apparatus are also provided.