Lithiation can be used to control the superconductivity in layered lithium iron selenide hydroxides, transferring iron from one layer (the lithium hydroxide layer) to the active layer (the iron selenide layer, filling up vacancies). Muon spin rotation can be used to probe the superconducting state of these new materials and we propose to study the effect of further control using cobalt substitution. This project is part of our efforts to find new ways of establishing superconductivity in layered iron-based materials that go beyond the methods developed over the last few years. Post-synthetic lithiation gives us a chemical means of altering superconducting properties within a single compound, or family of compounds.