Potassium fertilisation strongly affects growth and carbon partitioning of eucalypt on tropical soil. In addition, potassium fertilization could be of great interest in mitigating the adverse consequences of drought in planted forests, as foliar K concentrations influence osmotic adjustment, stomatal regulation and phloem loading. But little is known about the effect of potassium nutrition on phloem transport and on the interaction between K nutrition and water availability. In situ 13C pulse labelling was conducted on tropical eucalypt trees grown in a trial plantation with plots in which 37% of throughfall were excluded using home-made transparent gutters (-W) or not (+W) and plots that received 0.45 mol K m-2 (+K) or not (-K). Three trees were labelled in each of the four treatments (+K+W, +K-W, -K+W and -K-W), for one hour by injecting pure 13CO2 in a whole crown chamber. We estimated the velocity of carbon transfer in the trunk by comparing time lags between the uptake of 13CO2 and its recovery in trunk CO2 efflux recorded by off axis integrated cavity output spectroscopy in two chambers per tree, one just under the crown and one at the base of the trunk. The velocity of carbon transfer in the trunk was twice as high in +K trees as in -K trees, with no significant effect of throughfall exclusion except for one tree exposed to a more pronounced water stress. Our results suggest that besides reductions in photosynthetic C supply and in C demand by sink organs, the lower velocity under K deficiency is due to a lower cross section area of the sieve tubes, while an increase in phloem sap viscosity is more likely limiting phloem transport under drought.