A reactor suitable for synthesis of ammonia or methanol comprises a catalyst bed 36 equipped with heat exchange tubes 32 supported by means 28-31 or 40, 42 providing catalyst fillable space between the tubes over substantially their whole length. Preferably catalyst particles between the tubes and in an uncooled bed 36 downstream thereof constitute a single body of catalyst. The reactor is especially suitable for synthesis at low pressures such that the heat transfer area provided by the tubes is relatively low.