The characterization of different heteroatom functional groups by employing <
sup>
31<
/sup>
P NMR has been developed for almost 30 years. In this study, an in-depth investigation of this commonly used method has been accomplished for the analysis of pyrolysis oil. Several commonly used internal standards for <
sup>
31<
/sup>
P NMR have been examined by in situ monitoring. The results indicated that endo-N-hydroxy-5-norbornene-2,3-dicarboximide (NHND) is not stable after a long period of storage or experiment (>
12 hours), but both cyclohexanol and triphenylphosphine oxide (TPPO) can be used as internal standards if a long experiment or storage is required. The pyrolysis oil has also been investigated by both short time (16 hours) in situ monitoring and long time (14 days) ex situ monitoring. The results showed that aliphatic OH, carboxylic acids and water contents are not very stable after 2 hours, and thus a short time of preparation, storage, and experiment need to be considered to ensure a precise quantitative measurement. The decomposition products are still unclear, but some preliminary investigations for different acids, (e.g. formic acid) have been accomplished. The results indicated that the aromatic carboxylic acids (benzoic acid and vanillic acid) are more stable than formic acid and acetic acid. Interestingly, the formic acid will even decompose to some other compounds at the very beginning of the in situ monitoring test. Further characterization found that water is one of the major products for the decomposition of formic acid in the <
sup>
31<
/sup>
P NMR solution. Finally, as far as we know, this is the first report on such time-dependent changes when using <
sup>
31<
/sup>
P NMR to analyze the pyrolysis oil, and these results show that proper application of this method is essential to achieve reliable quantitative data.