A catalyst system for a liquid phase carbonylation reaction comprising a homogeneous acid catalyst component and a porous solid component, in particular for use in the formation of glycolic acid by carbonylation of formaldehyde. The homogeneous acid catalyst component is, for instance, sulphuric acid while the solid component can be unfunctionalized silica. A process for the carbonylation of an aldehyde to form a carboxylic′ acid or derivative thereof is also described. The process comprises the steps of contacting the catalyst with carbon monoxide, water and the aldehyde.