Significant changes occurred in the chemistry of gases discharged from Nisyros fumaroles in the years, 1997–2001, following a strong seismic crisis. Increasing H2S/CO2 ratios and decreasing CH4/CO2 ratios are attributed to an increased contribution of magmatic fluids to the hydrothermal system. Some fumaroles showed concurrent increases in H2 and CO contents implying increases in temperature and pressure in the upper parts of the hydrothermal system. These changes reinforce the possibility of an increased hazard of hydrothermal eruptions at present with respect to the '90's. Moreover, both the present events and the historical hydrothermal eruptions may represent precursory signals of a new period of volcanic unrest at Nisyros.