The extraction of caffeine from coffee beans are largely based on organic solvents such as dichloromethane, however, the water extraction process which represents a smaller portion in the worldwide production of decaffeinated coffee, with few studies on the transport phenomena, and mostly, the coffee beans are associated with a spherical geometry. The objective of this research was to determine the morphology that best fits the mass transfer phenomena in the decaffeination process. Aqueous decaffeination was conducted using previously hydrated coffee beans of Arabica variety, at temperatures ranging from 30 to 60 ºC. To determine the degree of approximation to hemispherical geometry, an equation based on a triaxial semi-ellipsoid was proposed. The results obtained by applying the analytical solutions of the Fick’s equation for the sphere and the hemisphere were compared. The coffee beans had an approximation to the hemisphere of 0,95 for dried grain (11,02% moisture on wet basis) and 0,92 for hydrated grain (53,94% moisture on wet basis). The reduction of the approximation is due to the increase in volume being greater than the increase of the surface area of the grains in the hydration process. The coefficient of diffusion of the caffeine found for the hemispherical model varied from 1,03·10-11 to 9,00·10-11 m2 s-1 for temperatures between 30 and 60 ºC, with the activation energy of 59,93 kJ mol-1 by the Arrhenius model. The hemispherical model presented correlation coefficients between 0,988 and 0,998 indicating an excellent fit to the experimental data.