Active volcanoes of Nicaragua, including Momotombo volcano, were monitored in 1982--1985 to determine volcanic hazard and to establish criteria for prediction of eruptions. In 1982 the chemical composition and isotopic ratios of fumarolic gases were found to depend on temperature. In the products of fumarolic activity the materials of crustal origin are of great significance. Sharp variations in the gas composition depending on exogenic factors were noted in 1983; the H2S/SO2 ratio was influenced the least. The temperature of fumarolic gases at Momotombo gradually increased from 740¿ C in 1974 to 895¿ C in 1984. During the same period the contents of H2S, SO2, CO, and H2 and the values of S/Cl, S/C, H2S/SO2, H2/H2O, and CO/CO2 ratios have risen, and the isotope ratios of hydrogen, sulfur, and especially carbon have changed. Future work in the crater of Momotombo will show whether the changes in the temperature and composition of the fumarolic gases are the precursors to a forthcoming eruption.