Activity coefficients in mixed electrolyte solutionsElectronic supplementary information (ESI) available: (Part A) 1. Apparatus and reagents. 2. Modus operandi. 3. Ion selectivity and water-induced ion selectivity. 4. Tests for ion selectivity (Tables S1–S5). 5. Less demanding checks. 6. Mixed electrolytes with (and without) ions in common. 7. Revised activity coefficients of [Co(en)3][Co(CN)6] in water at 298.15 K (Table S6). 8. Activity coefficients of reference mixed solutions (Tables S7–S15). 9. Activity coefficients of trace electrolytes in supporting electrolytes of variable concentration. (Tables S16–S24). 10. Ion hydration effects. (Part B). The IPBE algorithm: how to use the computation program; the source program IPBE. See http://www.rsc.org/suppdata/cp/b3/b311624g/