We show here a new reaction between lead(II) oxide and formaldehyde aqueous solution, which has been overlooked all along. The special structure of the new substance (PbCH2O2) and the DFT calculations suggest a diol-mechanism, which not only informs people about the corrosive nature of HCHO toward Pb and PbO, but also leads us to discover some new reactions between a variety of vicinal diol-type molecules and PbO. The reaction is further highlighted because of its potential application in detection and treatment of formaldehyde-containing wastewaters.