Key message <p>Osmotically-driven radial transfer of water from xylem to phloem occurs during the day in the branches of hinoki cypress, even though branch diameter shrinks.</p> Abstract <p>The hydraulic coupling between xylem and phloem has been explicitly included in theoretical models of phloem transport; however, the predicted radial water transfer from xylem to phloem during the day has not yet been demonstrated experimentally. In a first experiment, we tested the hypothesis of a water transfer from xylem to the inner bark during the day, assuming that, if the transfer does not occur, phloem water would be isotopically enriched due to an influx of enriched water concomitant with sugar loading in the foliage. The inner bark water from branches of mature hinoki cypress was, however, only slightly enriched in ¹⁸O in the afternoon compared to sapwood water (δ¹⁸O of −&#xa0;6.3 and −&#xa0;6.6‰ respectively), but much less than foliage water (16.0‰ at noon and 18.9‰ in the afternoon). This suggests that the isotopically enriched leaf water concurrently loaded with sugars in the leaf phloem was mixed with the unenriched xylem water. In a second experiment, to confirm that water transfer from xylem to the inner bark during the day occurs in branches, deuterium-enriched water was sprinkled on the soil. Concomitant deuterium enrichment of xylem and inner bark water extracted from branches was observed during the day after labelling, earlier than in leaves, when branches were still shrinking. This provides experimental support for theoretical models of phloem transport that have predicted radial transfer of water from xylem to phloem along the path.</p>