The practical procedure to design and optimise the FGD in-duct injection processes at low temperatures requires the application of the kinetic rate equation of the reaction SO2-Ca(OH)(2) in the mass balances of gas and solid phases corresponding to the ductwork, the sorbent utilization ability being an important parameter in the kinetic description. The objective of the study was focused on the behaviour of Ca(OH)(2) working under different levels of the process variables, temperature, relative humidity and water vapour content in the gas stream, in order to evaluate their influence on the sorbent utilization in the desulfurization reaction. The experimental data were obtained in an integral fixed bed reactor, monitoring continuously the SO2 content in the exit gas stream until the breakthrough curve was completed. The solid product was analysed by a thermogravimetric technique to calculate the final sorbent utilization. From the TG analysis, the contributions of the sulfation and carbonation reactions were established in terms of solid conversion: X-S, mol of SO2 mol of reacted sorbent and X-C, mol of CO(2)mol of reacted sorbent, and X, total sorbent utilization considering the amounts of both calcium sulfate and calcium carbonate in the reaction product. A detailed planning of experiments was followed: three different temperature levels were investigated: 54, 60, 70 degrees C, while the range of relative humidity was varied between 40 and 95%. The composition of the gas phase was kept constant (4000 ppmvSO(2), 12% CO2, 5% O-2, N-2 balance) as well as the amount of sorbent introduced in the reactor. The experimental data obtained were evaluated and the statistical significance of the effects of relative humidity, temperature and their interactions, was tested through an ANOVA table. From the sorption results, it was seen that there was a slightly negative influence of the reaction temperature while the relative humidity dependence was stronger at higher levels due to the sulfation reaction, with X-S values increasing from 0.05 to 0.50 when the relative humidity increased from 40% to 95%. The contribution of the carbonation reaction to the solid utilization was not affected significantly by the relative humidity or temperature. The carbonation fraction of reacted solid, X-C, reached values of around 0.20 in the whole experimental range.