In this work, we report on a novel method for the determination of aromatic aldehyde monomers (i.e., p-hydroxybenzaidehyde (PA), vanillin (VA), and syringaldehyde (SA)) in the lignin degradation liquors. The method uses a Nafion membrane to separate aromatic monomers from the samples. The monomers are then analyzed spectrophotometrically over the wavelength range of 343-415 nm, where only PA, VA and SA have absorptions. Multivariate numerical analysis is then used to solve the overlapping spectra of these three monomers in this wavelength range. It was found that the Nafion membrane separation system is effective in minimizing most interfering species (mainly, the larger molecular weight lignins). The closed-loop flow collection apparatus in the membrane separation procedure enhances the concentration of these monomers in the acceptor solution, so that they can be measured more easily via ultraviolet (UV) spectroscopy. The method requires no sample pretreatment and is simple, accurate, and suitable for analysis of the aromatic aldehyde monomers that are common in lignin degradation research.