We have demonstrated multistep topochemistry of the oxide chalcogenides Sr2MnO2Cu1.5S2 and Sr2MnO2Cu1.5Se2 via the lithiated reactive intermediates leading to conversion of their copper chalcogenide slabs into 2D arrays of chalcogen dimers (Ch2) 2-with perovskite-type Sr2MnO2 slabs retaining structural integrity. This is a chemical complement to electrochemical cycling previously carried out for the related oxysulphide Sr2MnO2Cu3.5S3. This chemistry highlights redox competition between cations and anions. Mn 4+ is common in oxides such as MnO2, while MnS2 contains oxidised (S2) 2-species, due to Mn 2+/3+ bands lying below the top of the S 3s/3p bands. In Sr2MnO2Cu1.5S2, heteroleptic MnO4S2 octahedra experience oxidation of Mn during partial Cu deintercalation using I2, but further Cu deintercalation giving the "collapsed" phase described here involves predominantly oxidation of chalcogenides, so Mn 2+/3+ cation redox is in tight competition with S 2-/-anions redox in the oxychalcogenides. This