Reduced X-ray reflectometry dataset associated with Diamond Light Source experiment SI10526-1, and neutron reflectometry dataset associated with Institut Laue-Langevin experiment 9-13-612, used in the publication "Bayesian determination of the effect of a deep eutectic solvent on the structure of lipid monolayers". The datasets are associated with the X-ray reflectometry of DLPC, DMPC, DPPC, and DMPG at the interface between a 1:2 mixture of choline chloride:glycerol and air, and the neutron reflectometry of DMPC and DPPC at the same interface (two isotopic contrasts).