We investigate the heterogeneously catalysed oxidation of HX (X=Cl, Br and I) on the RuO2 (1 1 0) surface with DFT. We also solve a micro-kinetic model of HX oxidation and compare oxidation activity at different coverages. We further establish linear energy relations for the reaction intermediates over a range of different rutile oxide surfaces. Based on the scaling relations, two descriptors are identified that describe the reactions uniquely. By combining scaling with the micro-kinetic model, activity volcanoes for the three different oxidation reactions are derived. It is found that the commonly used RuO2 catalyst for HCl oxidation is closest to optimal for all three oxidation processes.