Adsorption of five l-arabinose tautomers, one acyclic and four cyclic (α, β, pyranose, and furanose) species, on a ruthenium surface was studied as a precursor process of the, nowadays more and more, industrially important sugar catalytic hydrogenation on metal surfaces in water medium. The study was mostly referred to a 37 atom metal catalyst fragment even though border effects on the adsorption processes were also checked employing a 61 atom metal fragment. To figure out conformational effects on the title process, the tautomer flexibility was, at first, investigated by the genetic algorithm based code Balloon considering the conformational spaces of the different aquo tautomers. On the whole, 30 l-arabinose conformers, representing the complete conformational set (of a realistic water solution), were isolated by the genetic algorithm based code Balloon. These were further refined at density functional theory (DFT) level and then were analyzed when interacting with a ruthenium surface, always at DFT level, by SIESTA. It was found that (1) cyclic l-arabinose tautomers give rise to less strong adsorption than the acyclic tautomeric form; (2) l-arabinose molecules preferentially adsorb perpendicularly to the metallic surface; (3) one among the α-pyranose and one among the β-furanose derivatives are largely the most abundant adsorbed species; (4) the dominant l-arabinopyranose and l-arabinofuranose surface configurations are clearly related to corresponding not-adsorbed species that preserve both conformations and intramolecular hydrogen bonds during their adsorption. The consideration of the points above allowed us to pick out significant properties characterizing l-arabinose adsorption on ruthenium.