We find a first-order correlation between the Sr and O isotope compositions of hydrothermal fluids and spreading rate at oceanic spreading centers. The isotope signatures of hydrothermal fluids at slow-spreading ridges are more rock dominated than those at fast-spreading ridges, implying that they have undergone a greater extent of Sr and O exchange with the crust. The local spatial and temporal variability in Sr and O isotope signatures within individual hydrothermal sites can be attributed to short-term variations in the hydrothermal system in response to magmatic or tectonic events, phase separation at depth, and mixing with seawater in the upwelling zone. However, these processes cannot explain the systematic global relationship of an increasing rock signature in hydrothermal fluids with decreasing spreading rate. A greater extent of Sr and O exchange between rocks and the circulating fluids can be achieved by (1) a longer fluid-flow pathway through the oceanic lithosphere, (2) more fractured rock that exposes a greater reaction area, (3) an increased residence time of the fluid due to slower hydrothermal circulation, or (4) a combination of all three. Our preferred interpretation is that fluid-flow paths are longer, and depths of penetration are likely greater, at slow-spreading ridges.