Recently developed laser-based techniques have been used to make in situ measurements of the δ18O values of hydrothermal vein quartz from the Tayoltita and Guanajuato mining districts of Mexico. The δ18Oqtz values within a single 2 x 5 cm sample of banded quartz from Guanajuato varied from 12.6‰ to 17.3‰. Similarly, δ18Oqtz values measured in a 2 x 3 cm sample from Tayoltita ranged from 5.7‰ to 9.5‰. In both cases, the spread of δ18O values measured within individual samples is comparable to the total ranges of δ18O values previously reported for vein samples from the respective districts. Fluid-inclusion homogenization temperatures measured for these samples rule out the possibility that the changes in the δ18O values observed could have been caused by temperature fluctuations; therefore, they must be the result of large shifts in the isotopic composition of the vein fluids.