The pertinent equations governing proton incorporation in oxides with special emphasis on high temperature proton conductors (HTPCs) are reviewed. The prototypical compound SrCe0.95Yb0.05O3-alpha is considered in detail. Different domains in the key parameters p(H2O) and p(O2) can be distinguished, where essentially one defect type balances the dopant concentration in the electroneutrality condition. A predominance diagram is presented where these domains are outlined. The transitions between the domains occur at critical pressures p*(H2O), p*(H2), p**(H2), p*(O2) and p**(O2) which are given by the equilibrium constants and the dopant concentration. By introducing new dimensionless variables such as p(H2O)/p*(H2O) it is shown that the whole process of proton incorporation can be described by one master curve which displays Sieverts-type behavior at low pressures and saturation at higher pressures of H2O. A similar diagram as above but incorporating conductivity data is also presented.