In Nature, L-proline (Pro) is a ubiquitous and highly effective osmolyte protecting cells against osmotic stress. To understand this effect, knowledge of the hydration of Pro and its interactions with dissolved salts is essential. We studied these properties by combining statistical mechanics and broadband dielec. spectroscopy and found that Pro remains strongly hydrated up to high amino acid concns. This is also the case upon NaCl addn. to a 0.6M Pro soln. Here, addnl. a Pro·NaCl aggregate is formed with a stability const. of K° ≈ 0.95...1.25 M-1, where Na+ and Cl- cooperatively bind to adjacent carboxylate-oxygen and ammonium-hydrogen atoms, resp.