In this investigation mathematical modelling has been done for the reaction between solid spherical zinc titanate pellets and hydrogen sulfide, H2S at high temperatures (400-700 degrees C). In this model, a second order equation which is obtained for the H2S concentration in the pellet is solved numerically using the 'finite difference' method. In this equation, the value of effective diffusivity of H2S is determined as a function of porosity and tortuosity. Besides this, the value of porosity is also found as a function of the overall conversion. It is assumed that the pellet is formed from many of the same radius spherical grains on which the shrinking core model and the expanded grain radius are taken into consideration. An Arrhenius type of equation is developed for the diffusivity of H2S through the product layer in a grain using the experimental data of Lew et al. [S. Lew, A.F. Sarofim, M.F. Stephanopoulos, Modelling of the sulfidation of zinc titanium oxide sorbents with hydrogen sulfide, AlChE J. 38(8) (1992) 1161-1169] so these diffusivity values could be evaluated as a function of temperature. The model developed for the pellet was validated using the experimental data of Woods et al. [M.C. Woods, S.K. Gangwal, K. Jothimurgesan, D.P. Harrison, Reaction between H2S and zinc oxide titanium oxide sorbents. 1. Single-pellet kinetic studies, Ind. Eng. Chem. Res. 29 (1990) 1160-1167]. When the tortuosity was formulated as inversely proportional to the sixth power of the pellet porosity, an excellent agreement between the experimental and the predicted conversion values were obtained.