Photoelectron spectroscopy (PES) investigation of two widely used herbicide compounds MCPA and 2,4-D is reported. The GW calculations are shown to offer a good description of their electronic structure. There is excellent agreement between the experimental and theoretical energies of the HOMO states of 2,4-D and MCPA, which are at 8.7 eV and 8.5 eV binding energy, respectively. The low energy tail of the HOMO state extends to about 8 eV. The good agreement between the UPS (ultraviolet photoelectron spectroscopy) spectra and the simulation validates the proposed conformer. The presented gas phase data will help researchers to analyze data from liquid and diluted samples of these herbicide molecules and perform additional theoretical modelling. •The UPS spectra of two widespread herbicides MCPA and 2,4-dichlorophenoxyacetic acid have been obtained and analyzed.•A side-by-side comparison of the geometric and electronic structures is presented.•There is excellent agreement between the experimental and theoretical energies of the HOMO states of 2,4-D and MCPA, which are at 8.7 eV and 8.5 eV binding energy, respectively.