A reaction chamber for controlling endothermic or exothermic reactions particularly with fluidised catalysts is provided with a plurality of vertical heat-exchange bundles disposed within the chamber, each of the bundles comprising an upper and lower headers <PICT:0626664/III/1> <PICT:0626664/III/2> interconnected by a plurality of tubular elements, and having individually controlled means for introducing thereto or removing therefrom heat-exchange medium. In Fig. 1 a reaction chamber 1 containing a fluidised catalyst bed on grid 6 has hexagonal or square cross-section tubular heat-exchange bundles 5, each of which has individual valved feed and discharge lines 7, 8 to permit isolation of a particular bundle, while monorails 10 and opening 11 are provided in the free space above the catalyst bed for removal and replacement of a bundle. If the reaction is endothermic the heat-exchange medium may be introduced through line 8 and removed by line 7 or vice versa in exothermic reactions. Fig. 4 shows a construction in which the outlet line 8 passes through the lower header 14 to render removal easier. The bundles should be so arranged that no portion of the catalyst space is more than 4 ft. from one, and the tubes of the bundles should be so spaced as not to restrict catalyst flow, e.g. not closer than 0.4 inch nor occupying more than 65 per cent of the cross-sectional area of reactor. One or more tubes, preferably near the centre of a bundle may be larger than the others to facilitate an internal circulation by heat siphon. The heat transfer medium may be vaporising or non-vaporizing fluids such as water, diphenyl, gas oils, or low melting alloys. The reaction vessel may have a central tube with heat-exchange bundles in or around it, the fluidised catalyst being circulated up through the tube and down in the annular space or vice versa.