The following new KNO 3-I 2 hybrid cycle to produce hydrogen was devised. (9) 2 KNO 3+I 2→ 2 KI + 2 NO + 2 O 2 (10) 3 NO + 3 2 O 2→ 3 NO 2 (11) 3 NO 2 + H 2O → 2 HNO 3 + NO (12) 2 KI + 2 HNO 3 2F2 KNO 3 + I 2 + H 2 (3) H 2O → H 2 + 1 2 O 2 Since reactions (10) and (11) were well known in the industrial manufacture of nitric acid, thermochemical reaction (9) and electrolysis (12) were studied experimentally. In reaction (9), KNO 3 converts to Kl quantitatively, reacting with a small excess of I 2 at 1000 K. A carbon-cloth electrode and a platinized carbon-cloth electrode were used as an anode and a cathode, respectively, with small polarizations. Cation-exchange membranes were tested from the point of durability for electrolytes, permeation of I 2 and electric resistance. From the performance of an electrolyser, it was confirmed that a hot-press of the carbon-cloth anode with the membrane, a low ratio of I 2/I - in the anolyte and a high flow rate of the electrolyte solutions were effective to reduce the cell voltage below 1 V. The thermal efficiency of the cycle was estimated to assume a further improvement in the process.