An experimental study was performed on a countercurrent pilot-scale packed scrubber for wet flue gas desulfurization (FGD). The flow rate of the treated flue gas was around 300 Nm(3)/h, so the pilot-plant capacity is one of the largest with respect to other published studies on a pilot-plant wet FGD. The tests were carried out at an SO2 inlet concentration of 2000 ppm by changing the recycle slurry pH to around 4.8 and the L/G ratio to between 7.5 and 15. Three types of limestone were tested, obtaining desulfurization efficiencies from 59 to 99%. We show the importance of choosing an appropriate limestone in order to get a better performance from the FGD plant. Thus, it is important to know the reactivity (on a laboratory scale) and the sorbent utilization (on a pilot-plant scale) in order to identify if a limestone is reactive enough and to compare it with another type. In addition, by using the transfer-unit concept, a function has been obtained for the desulfurization efficiency, using the L/G ratio and the recycle slurry pH as independent variables. The Ca/S molar ratio is related to these and to the SO2 removal efficiency. This function, together with a simplified function of the operation variable cost, allows us to determine the pair (L/G ratio and pH) to achieve the desired SO2 removal with the minimum operation cost. Finally, the variable operation costs between packed towers and spray scrubbers have been compared, using as a basis the pilot packed tower and the industrial spray column at the Compostilla Power Station's FGD plant (in Leon, Spain).