I am having exactly the same problem with calculating Eh/pH diagrams for Fe-Se sulfides with act2. In fact, I am not titally convinced yet that there is no way to derive a unique solution. In my case, the Se concentration is 5 orders of magnitude below that of Fe and S, hence the influence of Se equilibrium on Fe and S speciation might be negligible. Hence in principle one should be able to derive the Fe-S stability fields first, and then speciate the Fe-S output with Se. Has somebody found a solution to this problem (which is rather common, I assume)?