This study investigated the relationship between ¹⁸O and ²H isotopes in samples of Mexican hair and drinking water. The purpose of this study was twofold: to quantify the relationship between isotopes in Mexican hair and tap water, in order to understand the impact of water stress and differing socioeconomic status on accurate predictions of drinking water; and to determine whether currently existing semimechanistic models can accurately represent the relationship between hair and tap water. This study used a subset of paired samples of human hair (n = 62) and tap water (n = 76). Isotope values in tap water ranged from –11.4‰ to –4.3‰ and –79.1‰ to –22.5‰, and in hair from +9.5‰ to +16.1‰ and –90.8‰ to –53.7‰, for δ¹⁸O and δ²H, respectively. The most depleted δ¹⁸O and δ²H hair values came from individuals in the state of Morelos. For modern Mexican populations, positive correlations between isotopes in hair and water were not significant, with correlation coefficients r = 0.61 (p = 0.05) and r = 0.60 (p = 0.06) for ¹⁸O and ²H, respectively. Error-in-variables regression yielded linear fits that were somewhat better for ²H relative to ¹⁸O: δ¹⁸O_h = 0.183 [±0.132] δ¹⁸O_tw + 15.7 [±0.9]‰ (r² = 0.23); δ²H_h = 0.181 [±0.076] δ²H_tw – 64.0 [±3.0]‰ (r² = 0.34). In short, data from this Mexican population did not exhibit the strong relationships between isotope values of ¹⁸O and ²H in tap water and hair that have been characteristic of other populations studied to date. Given the economic stratification of this region and the poor correlation between hair and water samples, the authors considered the possibility that l, the fraction of the diet derived from local sources, and f_s, the fraction of nonexchangeable H in keratin that was fixed in vivo, are local rather than global parameters for this population. The authors estimated different values of l and f_s for each location. Given the anticipated importance of the nonlocal dietary contribution, they treated the isotopic content of nonlocal food and the offset parameters for predicting isotopes in locally derived food as tuning parameters and compared the results with parameters based on the American supermarket diet. They found that, although O and H isotopes in water and hair maintained similar geographic distributions, O and H isotopes in tap water explained only a small part of the variation observed in hair samples. Compared to the standard American supermarket diet, the Mexican estimates for nonlocal diet and local diet offsets predicted regional distributions of l and f_s that cleanly segregated urban areas from rural towns.