In the near future, biogas is a promising alternative to fossil fuels to provide energy and heat. MILD oxy-fuel combustion is a recently proposed idea to utilize biogas more efficiently. So far there have been two possible ways to establish MILD oxy-fuel combustion: the reactants are diluted by CO2 or H2O, respectively. Although until now there have been a number of studies to compare reaction characteristics of biogas under these moderation operations, further investigation on this topic is still required. Especially, comparison from the viewpoint of the second law of thermodynamics has not received much attention. In this work, through changing the preheated temperature of oxidant and concentration of oxygen in oxidizer mixture, we carry out a comprehensive comparison on the performance of biogas MILD oxy-fuel combustion organized in O-2/CO2 or O-2/H2O atmosphere, respectively. The results show that, although H2O moderation has some advantages that summarized in previous publications, such operation also has a number of disadvantages, for example being high sensitive to oxygen concentration and existing large obvious temperature fluctuation, as compared with its CO2 counterpart. Some shortcomings of H2O moderation operation (e.g. worse performance due to more entropy generation), which have been neglected by previous research, can be more easily illustrated from the viewpoint of the second law of thermodynamics. Through the present analysis, it indicates that further comparison is necessary for other fuels to determine a suitable moderation approach to organize MILD combustion regime. (C) 2015 Elsevier Ltd. All rights reserved.