A staged steam hydrocarbon reformer is disclosed having a chamber within which convectively heated reformer stages are enclosed. The reformer stages are tubes containing steam reforming catalyst. The stages are in serial fluid communication with one another through mixing vessels positioned between each stage. The first reforming stage is fed a mixture of steam and a gaseous hydrocarbon. Partially reformed gases having increased hydrogen concentration are produced at each stage and are mixed with additional gaseous hydrocarbon and optionally steam in the mixing vessels. Collection and distribution manifolds provide fluid communication between the reformer stages and the mixing vessels. A method is also disclosed in which partially reformed gases from a preceding stage are mixed with gaseous hydrocarbon and steam having a lower steam to carbon ratio than the fresh feed to the previous stage.