The use of organic solvents in biomass conversion reactions can lead to high rates and improved selectivities. Here, we elucidate the effects of organic solvent mixtures with water on the kinetics of acid-catalysed dehydration reactions of relevance to biomass conversion. Based on results from reaction kinetics studies, combined with classical and ab initio molecular dynamics simulations, we show that the rates of acid-catalysed reactions in the liquid phase can be enhanced by altering the extents of solvation of the initial and transition states of these catalytic processes. The extent of these effects increases as the number of vicinal hydroxyl or oxygen-containing groups in the reactant increases, moving from an alcohol (butanol), to a diol (1,2-propanediol), to a carbohydrate (fructose). We demonstrate that the understanding of these solvation effects can be employed to optimize the rate and selectivity for production of the biomass platform molecule hydroxymethylfurfural from fructose.