The objective of this paper is to investigate the sources of volumetric irreversibilities in both laminar and turbulent diffusion flames. The theoretical background of analysis relies on the local exergy transport equation, which allows the microscopic formulation of the well-known Gouy-Stodola theorem. For laminar reacting flows, the volumetric entropy generation rate expression includes the viscous, thermal, diffusion and chemical components. Their expressions show that the corresponding irreversibilities are uncoupled if the combustion process occurs at constant pressure. The numerical simulation of a methane-air combustion process shows that the thermal, chemical and diffusive irreversibilities represent, in order of enumeration, the predominant irreversibilities in the laminar diffusion reacting flows. In the case of turbulent diffusion flames, the viscous, thermal, diffusion and chemical mean components have to be expressed in accordance with the combustion model. Two combustion models are used: the multi-species approach based on the eddy-break formulation of mean reaction rate, and the assumed probability density function for a conserved scalar that relies on the flame sheet model. For a diffusion methane-air jet flame, the distribution of mean irreversibility components is presented. Taking into account the technical importance of diffusion flames, the analysis could serve to improve the combustion geometry and the flow condition.