RATIONALE The condition of the pyrolysis reactor is very important for obtaining stable, precise hydrogen isotopic ratios using gas chromatography/thermal conversion/isotope ratio mass spectrometry (GC/TC/IRMS). However, few studies of the conditioning process have been conducted, and little is known about the best methods for high-precision hydrogen isotope analysis. METHODS We investigated delta H-2 variations and observed the changes in carbon coating using six different conditioning methods for the pyrolysis alumina tube: (i) no treatment; (ii) conditioning with 4 mu L hexane; (iii) conditioning with 2 mu L hexane; (iv) conditioning with 2 mu L hexane followed by backflushing overnight; (v) conditioning with 10 s of backflushing with methane; (vi) conditioning with 3 s of backflushing with methane. RESULTS Conditioning the alumina tube can improve the pyrolysis efficiency of organic compounds because a moderate amount of carbon acts as a catalyst in high-temperature regions of the alumina tube. Carbon actually flows in the tube and is difficult to confine to the high-temperature region. Insufficient amounts of carbon in the high-temperature regions lead to incomplete pyrolysis of organic compounds and lower delta H-2 values due to kinetic fractionation of hydrogen isotopes. In contrast, excess hexane or methane can lead to higher delta H-2 values, probably due to enrichment of deuterium in the hydrocarbon residue. CONCLUSIONS The delta H-2 values obtained by Method 6 are closest to the TC/EA delta H-2 values and are more precise than those obtained by other methods, perhaps because this method introduces a moderate, consistent amount of carbon with each sample injection.