Phenol was detected for the first time in the mononitrobenzene production process by benzene adiabatic nitration, supporting the theory that proposes phenol as the precursor of nitrophenols, unwanted byproducts. For that, analytical procedures (based in liquid and gas chromatographic techniques) have been developed and improved that aim to detect these chemical species both in the industrial nitration process as well as in laboratory tests at different reaction conditions. The obtained results confirmed the proposed and accepted mechanism for the nitrophenols formation during benzene nitration, showing that phenol is indeed the nitrophenolic byproducts precursor. Moreover, it is shown the phenol's high reactivity even at smooth reaction conditions (namely low temperature and low sulfuric acid concentration), which explains the difficulty of detecting this compound within the reaction medium using the analytical techniques applied for monitoring benzene nitration process. In this study, it was also discovered that 4-mononitrophenol had higher affinity into the aqueous acid phase when compared to the other nitrophenolic byproducts (2-mononitrophenol; 2,4-dinitrophenol; 2,6-dinitrophenol; and 2,4,6-trinitrophenol) that have more affinity into the organic phase. Phenol distribution between both reaction phases was found to be similar to the majority of the nitrophenols, i.e., it is present in higher concentration in the organic phase.