The total amount of residual free monomeric methylene diphenyl diisocyanate, fmMDI, in polyurethane foams and prepolymers has been lowered by EU and US EPA legislation due to their harmful effects on human health. A new HPLC method using either UV or mass spectrometry detection has been developed for quantitative analysis of fmMDI. The isocyanate group is stabilized with a new derivatizating amine, N-benzylmethylamine (NBMA), resulting in analytes that are considerably more soluble in solvents routinely used in HPLC than previous methods. The naphthyl isocyanate-NBMA derivative is used as an internal standard, either in the calibration curve method or the standard addition method. Mass spectra of the three compounds of interest (NBMA, fmMDI NBMA derivative, and naphthyl isocyanate NBMA derivative) have been obtained and structural assignment of the major fragment peaks has been accomplished, thus establishing relevant parent-daughter mass transitions in ESI + -MS/MS mode that made the quantitation process highly specific for NBMA derivatives of fmMDI isomers. UV detection uses a wavelength of 254 nm. The HPLC separation is accomplished by columns packed with 5 mm reversephase C18 particles. Intra-and inter-day precision for both methods are below the limits established by the Horwitz equation, and the accuracy was measured at 108-111% recovery for both methods. The method does not use toluene, xylene, DMF, DMSO or chlorinated compounds. The use of MS/MS detection considerably reduces the possibility of false or grossly inflated results, avoiding errors that are common in GPC-UV underivatized measurements.