A time series study of four hot spring fields at 21 ¿N on the East Pacific Rise demonstrates that the solution compositions of mid-ocean ridge hydrothermal vents are stable on a decadal time scale. The Ocean Bottom Seismometer (OBS), National Geographic Smoker (NGS), and South West (SW) vents show no significant changes in the major element composition of their end-member fluids since 1979. Two of the vents, OBS and SW, have maintained exit temperatures near 350 ¿C since 1979. The NGS Gardens, shows important changes in solution chemistry from 1981 to 1985. In 1985 this vent appears to have cooled ~10 ¿C relative to the other vents. The depth of the reaction zone, as determined by quartz geobarometry, has increased by about 1 km at this vent. The large fluxes of compositionally stable fluids from these vents cannot be explained by transient conditions and unique circumstances. Rather, the constancy of small interfield variations in solution composition is more consistent with control by rock buffering. A 4-year time series study of hydrothermal vents in the Guaymas Basin also shows no significant changes in the major element composition of the end-member fluids. Interfield differences in solution chemistry have been maintained from 1982 to 1985. This phenomenon is best explained in terms of an equilibrium model. ¿ American Geophysical Union 1988