We have re-assessed the main physicochemical features of the hydrothermal system of La Soufrière of Guadeloupe (FWI) andesitic volcano. A careful analysis of different techniques adopted historically for gas sampling and analysis by OVSG has allowed us to extend the use of our modeling and of gas thermobarometers as back as possible to the last 20 years, also including data from the 1976-77 phreatic eruption. Our aim was to track the evolution of P-T conditions of the gas equilibrium zone within the hydrothermal system. Our results show that long-term fluctuations characterize the behavior of the hydrothermal system in relation to injections of more magmatic deeper-sourced fluid into the hydrothermal system. This oscillations may lead to P-T excursions around and over the critical point of water, which destabilize the hydrothermal system. Whether such long-term increases reflect an oscillating behavior of the deep source injecting fluids upward, or are due to the modulation determined by the volcano structure via the many structures dissecting the edifice and relaxing the accumulated tensions, is matter of the ongoing investigations. Nevertheless, available data show that the recent unrest phase recorded between February and the end of April 2018 (Moretti et al., submitted; see also OVSG-IPGP reports) was due to the temperature increase and pressure build-up of the hydrothermal system, which originated a rapidly occurring (in the order of days) but sharp peak in monitored geochemical quantities. Therefore, scenarios that could lead to the sudden decompression of critical fluids must be considered in monitoring strategies and risk analysis.