This compressed file is supplementary to the article 'Controlling and predicting alkyl-onium electronic structure'. Its contents are two-fold: (1) Experimental X-ray photoelectron spectroscopy data of all ionic liquids studied in the article, in .vms format which can be accessed using CasaXPS and (2) The output files of all the ionic liquids studied computationally. These computational files will allow you to replicate the lone-ion-SMD (Solvation Model based on Density) Density Functional Theory (DFT) calculations using Gaussian 16 shown in the article.