Adsorption/desorption and pore-transport are key parameters influencing the activity and product selectivity in porous catalysts. With conventional reaction media (gas or liquid phase), one of these parameters is generally favorable while the other is not. For instance, while desorption of heavy hydrocarbons from the catalyst is usually the rate-limiting step in gas-phase reactions, transport of the reactants/products is the limiting step in liquid-phase reaction media. With conventional media, it is difficult to achieve the desired combination of fluid properties for optimum system performance. In contrast, density and transport properties can be continuously pressure-tuned in the near-critical region to obtain unique fluid properties (e.g. gas-like transport properties, liquid-like solvent power and heat capacities), that have been exploited in several ways such as (a) the in situ extraction of heavy hydrocarbons (i.e. coke precursors) from the catalyst surface and their transport out of the pores before they are transformed to consolidated coke; (b) complete miscibility of reactants such as hydrogen in the reaction mixture and enhanced pore-transport of these reactants to the catalyst surface, thereby promoting desired reaction pathways; and (c) control of temperature rise in exothermic reactions. Experimental and theoretical investigations are presented to demonstrate beneficial pressure-tuning effects on catalyst activity and product selectivity during continuous processing of a variety of reactions such as these: geometric isomerization and alkylation on solid acid catalysts; Fischer-Tropsch (FT) synthesis on supported Fe catalysts; and fixed-bed hydrogenation on supported catalysts. The possibility to perform solid acid catalysis with extended activity tan environmentally safer alternative to liquid acid processes) and fixed-bed hydrogenations with tunable selectivity and controlled temperature rise (preferred over slurry phase operation) makes supercritical reaction media particularly appealing alternatives to conventional reactor operation.