Hydration structures in the vicinity of solute arginine molecules have been investigated by measuring the mass spectra of protonated arginine-water cluster ions ejected from a liquid beam of an arginine aqueous solution by IR-laser ablation. It is revealed that an arginine aggregate is less hydrated than an arginine monomer, because of a smaller dipole moment, and that an arginine monomer is less hydrated in a concentrated arginine solution, because of formation of a hydrophobic hydration structure of water around an arginine aggregate in the solution. (c) 2005 Elsevier B.V. All rights reserved.