This deposition contains the data and analysis (Jupyter notebooks) detailed in "Underscreening in concentrated electrolytes: Polyelectrolyte brushes exhibit re-entrant swelling behaviour". All Jupyter notebooks have also been converted into PDF files for ease of viewing. All data and code (notebooks) required to reproduce the analysis can be found within the “supporting_data.zip” archive. This archive contains three sub-directories: Spectroscopic_ellipsometry Data directory (data_files) containing all raw ellipsometry data. “PMETAC_wet_analysis.ipynb” notebooks to reproduce the analysis of a hydrated (solid-liquid) polymer brush. Further relevant plotting tools can be found in the refellips repo. “PMETAC_dry.ipynb” notebook to reproduce the analysis of the dry polymer film and create the areal maps. Neutron_reflectometry Data directory (NamedData) containing all relevant reduced reflectivity profiles from the Platypus reflectometry at ANSTO. “NR_dry.ipynb”, “NR_Solid-Liquid.ipynb” notebooks required to reproduce the analysis pertaining to a dry polymer brush and a solvated brush, respectively. “NR_Solid-Liquid_corefine.ipynb” notebook for reproducing the simultaneous optimisation of the PMETAC brush in D2O and H2O: co-refinement. Additional code required to model the hydrated polymer brush and various plotting tools can be found in the refnxtoolbox repo and the refnx repo. nSCFT Files containing the polymer, ion, and solvent VF profiles from nSCFT calculations.