An exploration of the transient state of Rayleigh convection is implemented via the entropy generation analysis, which is always used to identify the irreversibility with the available energy losses for the non-equilibrium thermodynamic processes of CO2 absorbed into some solvents. The calculation of local entropy generation rate for the convective mass transfer is worked out with the quantitative Schlieren technology, and its profiles perpendicular to the gas-liquid interface are applied to analyze the evolution of Rayleigh convection. The results show that entropy generation reaches approximately 2.50 W K-1 m(-3) for the CO2-ethanol system until interfacial convection occurs to reduce local irreversibility with the synergistic effects of velocity vector and concentration gradient, as a result of providing a quantified thermodynamic criterion for predicting the onset of Rayleigh convection. Moreover, the critical value caused by mass transfer is determined by the density differences and boundary conditions for different solvents. The variation of global entropy generation shows that Rayleigh convection changes the irreversible route from a linear to non-linear thermodynamic branch with the emergence of the ordered plume-like structures. The detailed entropy generation analysis for the onset and development of convective cells is discussed to investigate Rayleigh convection and deepen the comprehension for the internal mechanism of convection enhancing mass transfer from the viewpoint of irreversible thermodynamics. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.