Abstract Portions of a bench-scale model of a sulfur-iodine thermochemical water-splitting cycle have been operated at General Atomic Company as part of a comprehensive program to demonstrate the technology for hydrogen production from non-fossil sources. The bench-scale model consists of three subunits which can be operated separately or together and is capable of producing as much as 4 l/min −1 (6.7 × 10 −5 m 3 s −1 ) at standard conditions of gaseous hydrogen. One subunit (main solution reaction) reacts liquid water, liquid iodine (I 2 ) and gaseous sulfur dioxide (SO 2 ) to form two separable liquid phases: 50 wt % sulfuric acid (H 2 SO 4 ) and a solution of iodine in hydroiodic acid (HI x ). Another subunit (H 2 SO 4 concentration and decomposition) concentrates the H 2 SO 4 phase to the azeotropic composition, then decomposes it at high temperature over a catalyst to form gaseous SO 2 and oxygen. The third subunit (HI separation and decomposition) separates the HI from water and I 2 by extractive distillation with phosphoric acid (H 3 PO 4 ) and decomposes the HI in the vapor phase over a catalyst to form I 2 and product hydrogen. This paper presents the results of ongoing parametric studies to determine the operating characteristics, performance, and capacity limitations of major components.