The components of the intracellular environment vary widely in size: from large multiprotein complexes to atomic ions. Besides water, low-molecular-weight solutes (<1 kDa) such as electrolytes, metabolites, and carbohydrates are by far the most abundant of these components. Small solutes are thus key contributors to the solvation environment in the cell. Small solutes have been known for decades to alter protein structure or activity in vitro, through their interactions with protein surfaces or hydration shells. Here we use the cell itself as our test tube, by titrating its hydration, ion, or carbohydrate composition systematically. We trigger the selective uptake of specific solutes by exposing cells to hyperosmotic media. We then measure protein structure, stability, unfolding kinetics, and aggregation in these different intracellular environments by using fast relaxation imaging. We compare these results with controls where solutes cannot enter the cell and only hydration is altered. Protein structure, thermal stability, and aggregation onset all depend on the concentration and chemical nature of the solute titrated into the cell. Our work highlights the important contributions of small solutes in defining how proteins interact within the cell and suggests that intracellular variation of the solute composition could be an important regulator of protein function.