A novel diode laser spectrometer was developed using dualwavelength multiplexing, ensuring ideal conditions for high-precision and simultaneous measurements of the 2H/1H, 17O/16O, and 18O/16O isotope ratios in water. A 1.4-m diode laser probed a H16OH / HO2H line pair near 7198 cm-1, while a similar laser observed H16OH, H17OH, and H18OH rovibrational lines around 7183 cm-1, or a H16OH / H18OH line pair near 7200 cm-1. The 1-sigma standard deviation is 0.2 per thousand for 18O/16O, and 0.5 per thousand for the 2H/1H and 17O/16O isotope ratios. Preliminary experiments with repeated injections of a natural abundance sample point to an accuracy of about 1 per thousand for all three isotope ratios in natural samples.