In this work, the concentration profiles of charged species in a flat, fuel-lean (Φ=0.216), and laminar methane–oxygen flame are calculated using a detailed transport model and an elementary reaction mechanism. For the first time, these simulations include negative ions besides neutral species and positive ions. The chemical reaction mechanism of the charged species is compiled from different sources found in relevant literature. Overall, the ion model contains 65 reversible reactions involving 11 charged species. Special emphasis is put on the diffusion processes of the ions. An ambipolar model is developed and discussed which describes the mutual interactions of the charged species during diffusion. It allows for an arbitrary fraction of negative ions because it does not depend on the assumption that the electrons dominate the diffusion process. Reaction flow analyses show the chemical pathways taken. Special focus is put on the effects of ambipolar diffusion on the species profiles and the importance of negative ions. Our findings are compared to experimental results published in the literature.