A process for the direct hydrogenation of hop resin acids in the absence of a liquid, organic solvent by reacting the hop resin acids which hydrogen in the presence of a noble metal catalyst at a temperature at which the resin acids are sufficiently fluid to allow easy mixing with the catalyst and suitably facilitate the hydrogenation reaction. Alternatively, the necessary fluidity may be achieved at relatively low temperature by subjecting the reactants to a high pressure of CO.sub.2 gas such that the quantity of this gas that dissolves into the resin is sufficient to substantially reduce its viscosity. Particularly useful applications for the process are the conversion is iso-.alpha.-acids to tetrahydroiso-.alpha.-acids and rho-iso-.alpha.-acids into hexahydroiso-.alpha.-acids. The resinous products of the hydrogenation process may be readily converted to aqueous, alkaline solutions suitable for use in brewing beer. The process may also be worked in a continuous manner, wherein hop resin acids in suitably fluid state and under high pressure are contacted with hydrogen gas and the resultant mixture passed over a bed of noble metal catalyst particles. The necessary fluidity is achieved either by heating the hop resin acid or by dissolving CO.sub.2 gas into it or a combination of both. The resinous product of the hydrogenation reaction is collected by depressurisation of the fluid exiting the catalyst bed, conveniently causing release of any excess dissolved gas.