The results of extensive cycle chemistry monitoring campaigns of ENEL (Italian Electricity Board) at Porto Tolle 4 and Porto Tolle 2 fossil fuel power plant are reported. The information acquired allows a detailed description of the chemistry through the cycle and a comparison of different water chemistries (AVT-P04 and CWT-P02) in once-through supercritical boilers. The feasibility of controlling cycle chemistry parameters within the limits suggested by the boiler manufacturer and the new EPRI Iterim Consensus Guidelines was confirmed for the AVT (All Volatile Treatment) conditioning. For CWT (Combined Water Treatment) data were compared for the VGB (German specifications) limits for similar boilers. The values and trends reported for the most significant chemical parameters, measured through the cycle, indicate that, by changing from AVT to CWT, a significant reduction of the oxides generation and transport to the boiler is achieved, as expected. Furthermore, the cycle chemistry appears reasonably fair or even better than that with AVT.