Rationale: Laser-based analyzers are widely used in ecohydrology to analyze plant water isotopic compositions (δ¹⁸O and δ²H). The suitability of three different water extraction and isotope equilibration techniques was compared. We examined whether co-extracted volatile organic compounds (VOCs) affect laser-based isotope measurements and used the instrument's spectral parameters to post-correct for interfering VOCs. Methods: Cryogenic vacuum extraction, vapor headspace equilibration in bags, and vapor equilibration in situ probes were used to extract liquid water or water vapor for laser-based isotope analysis (cavity ring-down spectrometry, CRDS). Isotope data were calibrated by standards for each method separately. Spectral parameters of the instrument, appropriate to identify spectral interferences with MeOH and CH₄, were identified and used for post-correction. Differences between the three methods and between the origins of the vegetables were identified by statistical tests. Results: VOCs were found in various amounts for the three different methods. They were co-extracted or co-equilibrated during the different extraction or equilibration methods. Correlation coefficients of isotope data and “CH₄” (spectral parameter) were 0.99 or better; however, slopes for δ¹⁸O were similar on different instrument types but different for δ²H. Our correction approach improved results and inter-comparability of the methods considerably without knowing the chemical composition of the plant sap. Conclusions: All three methods were sensitive enough to distinguish and resolve differences in natural abundance. Data quality was improved by the “CH₄ correction” approach but could probably be optimized by a plant species–specific correction. Standardized tools for contaminant removal or post-correction applications from manufacturers, in particular for vapor-mode analysis, are still needed.