Stable hydrogen isotope ratio measurements of specific plant components are increasingly used in numerous fields of research, including sample origin verification and climate research. A recently suggested method with considerable potential in this context is the D/H isotope ratio (delta(2)H value) analysis of plant matter methoxyl groups. The method entails ether or ester cleavage with hydriodic acid (HI) to form the gaseous compound methyl iodide (CH(3)I) and measurement of the delta(2)H value of this gas. Here we describe a method for the rapid and precise delta(2)H analysis of plant matter methoxyl groups using gas chromatography/pyrolysis/isotope ratio mass spectrometry (GC/P/IRMS). The conditions for sample preparation were investigated for isotope discrimination effects, the GC conditions were optimized, the reproducibility of the measurement of standards was studied, and the precision of the method was defined. The reproducibility of delta(2)H values determined for a CH(3)I standard on 20 consecutive measurements was found to be 2 per thousand. The method was also tested on four methoxyl-rich plant components: vanillin, lignin, wood and pectin. The analytical precision obtained, when expressed as the average standard deviation for these compounds, was better than 1.6 per thousand. The described method is rapid, allowing preparation and analysis of a sample within 1 h, and produces accurate and reproducible isotopic measurements.