<p>Direct numerical simulations with multi-step chemistry were performed for one- and two-dimensional freely propagating laminar premixed flames of methane–air and hydrogen–air mixtures with a matching density ratio to isolate the effects of hydrodynamic instability while allowing for a variable effective Lewis number, with the methane (hydrogen) flame being thermodiffusively stable (unstable). Entropy diffusion and generation mechanisms were analysed based on contributions from heat conduction, viscous dissipation, mass diffusion, and chemical reactions. Across both flames, chemical reactions were identified as the dominant source of entropy generation, with viscous dissipation contributing negligibly compared to other mechanisms. Significant differences were found in the structure of entropy generation rates across both flames, with varying degrees of correlation with curvature. Stronger correlations were found between the irreversible entropy generation rates and the heat release rate in both flames, suggesting the former as a potential marker for thermodiffusive instability. Analysis of the entropy generation profiles at representative locations across a flame front further revealed possible origins of the entropy behaviour under thermodiffusively stable and unstable conditions.</p>