<p>The hydrogen isotopic composition of leaf wax-derived biomarkers, e.g. long chain <i>n</i>-alkanes (δ²H_{<i>n</i>-alkane}), is widely applied in paleoclimatology research. However, a direct reconstruction of the isotopic composition of paleoprecipitation based on δ²H_{<i>n</i>-alkane} alone can be challenging due to the overprint of the source water isotopic signal by leaf-water enrichment. The coupling of δ²H_{<i>n</i>-alkane} with δ¹⁸O of hemicellulose-derived sugars (δ¹⁸O_sugar) has the potential to disentangle this effect and additionally allow relative humidity reconstructions. Here, we present δ²H_{<i>n</i>-alkane} as well as δ¹⁸O_sugar results obtained from leaves of the plant species <i>Eucalyptus globulus</i>, <i>Vicia faba</i> var. <i>minor</i> and <i>Brassica oleracea</i> var. <i>medullosa</i>, which were grown under controlled conditions. We addressed the questions (i) do δ²H_{<i>n</i>-alkane} and δ¹⁸O_sugar values allow precise reconstructions of leaf water isotope composition, (ii) how accurately does the reconstructed leaf-water-isotope composition enables relative humidity (RH) reconstruction in which the plants grew, and (iii) does the coupling of δ²H_{<i>n</i>-alkane} and δ¹⁸O_sugar enable a robust source water calculation?</p> <p> For all investigated species, the alkane <i>n</i>-C₂₉ was most abundant and therefore used for compound-specific δ²H measurements. For <i>Vicia faba</i>, additionally the δ²H values of <i>n</i>-C₃₁ could be evaluated robustly. With regard to hemicellulose-derived monosaccharides, arabinose and xylose were most abundant and their δ¹⁸O values were therefore used to calculate weighted mean leaf δ¹⁸O_sugar values. Both δ²H_{<i>n</i>-alkane} and δ¹⁸O_sugar yielded significant correlations with δ²H_leaf-water and δ¹⁸O_leaf-water, respectively (r²&thinsp;=&thinsp;0.45 and 0.85, respectively; p&thinsp;<&thinsp;0.001, n&thinsp;=&thinsp;24). Mean fractionation factors between biomarkers and leaf water were found to be &minus;156&thinsp;‰ (ranging from &minus;133 to &minus;192&thinsp;‰) for ε_{<i>n</i>-alkane/leaf-water} and +27.3&thinsp;‰ (ranging from +23.0 to 32.3&thinsp;‰) for ε_{sugar/leaf-water}, respectively. Using rearranged Craig-Gordon equations with either T_air or T_leaf and measured δ²H_leaf-water or δ¹⁸O_leaf-water as input variables, we furthermore modeled climate chamber RH_air and RH_leaf values. Modelled RH_air values, from the more simplified Craig-Gordon model, turned out to be most accurate and correlate highly significantly with measured RH_air values (R²&thinsp;=&thinsp;0.84, p&thinsp;<&thinsp;0.001; RMSE&thinsp;=&thinsp;6&thinsp;%). When combining δ²H_leaf-water and δ¹⁸O_leaf-water values that are calculated from the alkane and sugar biomarkers instead of actually measured δ²H_leaf-water and δ¹⁸O_leaf-water as input variables, the correlation of modelled RH_air values with measured RH_air values is getting worse, but is still highly significant with R²&thinsp;=&thinsp;0.54, p&thinsp;<&thinsp;0.001; RMSE&thinsp;=&thinsp;10&thinsp;%. This highlights the potential of the coupled δ²H_{<i>n</i>-alkane}-δ¹⁸O_sugar paleohygrometer approach for suitable relative humidity reconstructions. Finally, the reconstructed source water isotope composition (δ²H_s and δ¹⁸O_s) as calculated from the coupled approach matches the source water in the climate chamber experiment (δ²H_tank-water and δ¹⁸O_tank-water).</p>