Abstract Pore diffusion limitations dampen the sensitivity of temperature runaway to variations in the coolant temperature in methanol synthesis reactors. For an isothermal pellet and low product concentration, the relationship between catalyst effectiveness and the Thiele parameter can be represented with a single curve. However, when equilibrium is approached or at high product concentrations, the relationship between catalyst effectiveness and the Thiele parameter becomes a function of catalyst surface temperature. For large temperature gradients in the catalyst, the effectiveness factor can significantly increase and parametric sensitivity in the relationship between catalyst effectiveness and the Thiele parameter may result.