Here we report the first in situ pH of hydrothermal vent fluids at mid-ocean ridges. Measurements were made during dives with DSVAlvin to the Main Endeavour Field (Juan de Fuca Ridge) and the East Pacific Rise at 21°N, and 9°–10°N using solidstate electrochemical sensors. Vent fluid temperature and pressure ranged from 180 to 384 °C and 220 to 250 bar, respectively. pH_{(in situ)} of the highest temperature vent fluid is only slightly acidic (5.1–5.4), although sharply lower values occur at the seawater/vent-fluid interface. Knowledge of pH_{(in situ)} is essential for unravelling the complex geochemical and biogeochemical processes controlling the evolution of seafloor hydrothermal systems.