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An iridium(I) N-heterocyclic carbene/phosphine complex has been applied to the C-H activation and hydrogen isotope exchange of quinoline N-oxides. The isotope labelling proceeds under exceptionally low catalyst loadings of 0.25 mol%, and delivers products with high levels of deuterium incorporation selectively at the C8 position. A broad substrate scope is demonstrated, with the method tolerant of electron-poor and -rich substrates, and of substitution adjacent to the site of C-H activation. The isotope label is fully retained under standard deoxygenation conditions to give the corresponding labelled quinoline, and the labelling and deoxygenation can be combined in a one-pot procedure.