A new type of gas solid reactor was developed and characterised in the series of reactor configurations with separate feed of reactants studied by our group. The novelty in the proposed design lies in the use of a fixed bed of small catalytic particles instead of a porous catalytic membrane. The major advantages of this concept, as opposed to the catalytic membrane reactor. are: (i) the barrier activity and pore structure are more homogeneous (good properties for modelling purposes): (ii) an easier integration of the catalyst bed into the reactor is obtained (the fixed bed can be sealed more easily into metallic modules than a catalytic membrane). A Pt catalyst supported on La-stabilised alumina pellets (d(p) = 5 similar to 50 mum) was used and the catalytic oxidation of carbon monoxide was studied as a model reaction to characterise the reactor. A mathematical model based on the Dusty Gas Model approach was conceived and a good agreement between calculations and experiments was obtained. This fixed bed barrier reactor may thus represent a quicker and easier opportunity, compared to membrana reactors, to check the concept of the separate-feed of reactants with new reactions and catalysts.