Germanium/silicon (Ge/Si) ratios in Himalayan hot-spring fluids show a wide range, 4–1000 μmol/mol. Equilibrium calculations with an ideal solid-solution model of germanium in quartz yield Ge/Sifluid ratios consistent with data from high-temperature reservoirs but do not fit lower temperature observations, in part because of kinetic constraints. A model of progressive Si loss via precipitation of Ge-poor quartz (Rayleigh distillation) can produce the extreme Ge/Sifluid ratios observed in the Himalayan and other systems. Small variations in Si loss, mixing with surface waters, and/or disequilibrium can produce the observed variability in Ge/Sifluid within a given hydrothermal system. The high level of Si loss (∼95%) required to reach extreme Ge/Sifluid ratios is consistent with reaction-path calculations for the evolution of a geothermal fluid from its reservoir to surface temperatures and implies that Ge/Si ratios can be a useful tracer of silica dynamics in hydrothermal systems.