A fundamental description of gas-liquid mass transfer with reversible consecutive reaction has been derived. The Higbie penetration theory has been used and numerical simulations were carried out for isothermal absorption. Although the model can be adapted to reactions of general stoichiometric and kinetic orders, results in this paper have been limited to unit orders only. The model has been applied for a wide range of process conditions to investigate the effect of reversibility of both reaction steps and the effect of the use of (partially) loaded solutions on the mass transfer characteristics. For consecutive reactions with both steps irreversible, the approximate solutions of Onda (1970, 1972) have been found to be sufficiently accurate (maximum deviation of 4.3 % for the penetration theory solution). It has also been shown that the overall enhancement factor can be regarded as the summation of the enhancement factors of the individual reaction steps. This has been quantitatively shown for the case where the first step is irreversible while the second is reversible. Finally, an approximate technique to determine infinite enhancement factors for reversible consecutive reactions has been given. This approximation is based on the method described by DeCoursey (1982). Deviations from numerical calculations for both loaded and unloaded solutions were found to be less than 1.3%. Part I of this paper deals with the case of equal diffusivities of the chemical species involved whereas the effect of unequal diffusivities on the overall absorption rate and enhancement will be dealt with in Part II.