Experimental values for the Gibbs ’free energy of mixing’ of solid solutions produced by electrochemical H insertion into a commercial synthetic gamma-MnO2 have been derived from published electrode potential measurements carried out in Leclanche electrolyte. Calculated values are based on the reasonable assumption that the solid solutions are thermodynamically ideal so that the Gibbs free energy of mixing can be estimated from the positional entropy of the inserted species. Comparison of the two sets of values provides strong evidence that the inserted H is present as two thermodynamically independent species H+ and e at least up to 40% fill. Additional insight is provided by a planar random insertion model. The model shows the development of microdomains of fully H inserted material at the higher levels of insertion but prior to complete insertion as is required to explain published experimental phenomena. The model also allows the calculation of the Gibbs free energy of mixing to be refined to take into account either the stabilisation of certain H(+)e pair types or restriction in the movement of the H+ and e species of pairs caused by adjacent pairs blocking pathways. The latter refinement gives very good agreement with the experimental data using one adjustable parameter namely a blocking efficiency of 50%.