Applications of high-precision isotope analysis of ecohydrological water samples such as soil and plant waters with infrared (IR) laser spectrometry have been hampered by spectral interferences of volatile organic compounds. In this study, three methods were tested to remove methanol (MeOH) and/or ethanol (EtOH) added to water for δ ²H, δ ¹⁸O, and ¹⁷O-excess analysis with Picarro L2140-i: Micro Combustion Module (MCM), pre-treatment with solid-phase extraction (SPE), and pre-treatment with simple MeOH/EtOH combustion (SMEC) method. The Picarro MCM was not able to combust the alcohols of high concentrations. Hydrophilic-Lipophilic Balanced (HLB) SPE could adsorb MeOH/EtOH at low concentrations (e.g., 0.1% EtOH). However, its adsorption capacity could be readily exhausted, and adsorbed water on the SPE adsorbents co-elutes and mix with a sample water. The SMEC can successfully combust highly concentrated waters (0.5% MeOH and 1% EtOH) at 450-500°C, but the alcohol combustion and oxygen isotope exchange with added O ₂ could significantly increase δ ¹⁸O values (up to 6-7‰) and decrease ¹⁷O-excess (up to 110 per meg) in the presence of a catalyst. A strategy of combining the two methods is proposed for high-precision isotope analysis of organic contaminated samples: post-analysis corrections of Picarro MCM data (δ ²H and δ ¹⁸O) and pre-treatment with SMEC method (δ ²H and ¹⁷O-excess). Overall accuracy and precision are ≤3 and ≤2 ‰ (δ ²H), ≤0.2 and ≤0.3‰ (δ ¹⁸O), and ≤20 and ≤47 per meg ( ¹⁷O-excess), respectively.