AbstractReactive transport softwares are today one of the cornerstones of environmental research. They contain multiphysics with very complex algorithms, including flow, transport, chemical, and sometimes heat transport, mechanical, and/or biological algorithms. Because of this complexity, some parts of these algorithms still have not been sufficiently studied. In this work, we focus on algorithms for activity correction, a specific subset of equilibrium chemistry algorithms. We show that the most used algorithm (the inner fixed‐point algorithm) or the most rigorous algorithm (the full Newton) might not be the most efficient, and we propose a new one, the outer fixed‐point algorithm, which is more robust and faster than other algorithms.