A method for isotope ratio analysis of water samples is described comprising an on‐line high‐temperature reduction technique in a helium carrier gas. Using a gas‐tight syringe, injection of 0.5 to 1 μL sample is made through a heated septum into a glassy carbon reactor at temperatures in excess of 1300°C. More than 150 injections can be made per day and both isotope ratios of interest, δ ² H and δ ¹⁸ O, can be measured with the same setup. The technique has the capability to transfer high‐precision stable isotope ratio analysis of water samples from a specialized to a routine laboratory task compatible with other common techniques (automated injection for GC, LC, etc.). Experiments with an emphasis on the reactor design were made in two different laboratories using two different commercially available high‐temperature elemental analyser (EA) systems. In the Jena TC/EA unit, sample‐to‐sample memory (single injection) has been reduced to ∼1% and high precision of about 0.1‰ for δ ¹⁸ O and < 1‰ for δ ² H has been achieved by a redesign of the glassy carbon reactor and by redirecting the gas flow of the commercial TC/EA unit. With the modified reactor, the contact of water vapour with surfaces other than glassy carbon is avoided completely. The carrier gas is introduced at the bottom of the reactor thereby flushing the outer tube compartment of the tube‐in‐tube assembly before entering the active heart of the reactor. With the Leipzig high‐temperature reactor (HTP) similar precision was obtained with a minor modification (electropolishing) of the injector metal sleeve. With this system, the temperature dependence of the reaction has been studied between 1100 and 1450°C. Complete yield and constant isotope ratio information has been observed only for temperatures above 1325°C. For temperatures above 1300°C the reactor produces an increasing amount of CO background from reaction of glass carbon with the ceramic tube. This limits the usable temperature to a maximum of 1450°C. Relevant gas permeation through the Al ₂ O ₃ walls has not been detected up to 1600°C. Copyright © 2004 John Wiley & Sons, Ltd.

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