Surface tethered polyelectrolyte brushes are scientifically interesting and technologically relevant to many applications, ranging from colloidal stabilization to responsive and tunable materials to lubrication. Many applications operate in environments containing multi-valent ions, media in which our scientific understanding is not yet well-developed. We synthesized high-density polystyrene sulfonate (PSS) brushes via surface initiated atom-transfer radical polymerization, and performed neutron reflectivity (NR) measurements to investigate and compare the effects of mono-valent Rb+ and tri-valent Y3+ counterions to the structure of the densely tethered PSS brushes. Our NR results show that in monovalent RbNO3 solution, the dense PSS brush retained its full thickness up to a salt concentration of 1 M, whereas it immediately collapsed upon adding 1.67 mM of tri-valent Y3+. Increasing the concentration of Y3+ beyond this level did not lead to any significant further structure change of the PSS brush. Our findings demonstrate that the presence of multi-valent counterions can significantly alter the structure of polyelectrolyte brushes, in a manner different from mono-valent ions, which has implications for the functionality of the brushes. (C) 2016 Published by Elsevier Ltd.