Arguably, the most striking geochemical distinction between Earth and the Moon has been the virtual lack of water (hydrogen) in the latter. This conclusion was recently challenged on the basis of geochemical data from lunar materials that suggest that the Moon's water content might be far higher than previously believed. We measured the chlorine isotope composition of Apollo basalts and glasses and found that the range of isotopic values [from -1 to +24 per mil (parts per thousand) versus standard mean ocean chloride] is 25 times the range for Earth. The huge isotopic spread is explained by volatilization of metal halides during basalt eruption-a process that could only occur if the Moon had hydrogen concentrations lower than those of Earth by a factor of similar to 10(4) to 10(5), implying that the lunar interior is essentially anhydrous.