Processes controlling plant carbon allocation among primary and secondary metabolism, i.e., carbon assimilation, respiration, and VOC synthesis are still poorly constrained, particularly regarding their response to stress. To investigate these processes, we simulated a 10-day 38°C heat wave, analysing real-time carbon allocation into primary and secondary metabolism in the Mediterranean shrub Halimium halimifolium L. We traced position-specific 13 C-labeled pyruvate into daytime VOC and CO 2 emissions and during light-dark transition. Net CO 2 assimilation strongly declined under heat, due to threefold higher respiration rates. Interestingly, day respiration also increased twofold. Decarboxylation of the C1-atom of pyruvate was the main process driving daytime CO 2 release, whereas the C2-moiety was not decarboxylated in the TCA cycle. Heat induced high emissions of methanol, methyl acetate, acetaldehyde as well as mono-and sesquiterpenes, particularly during the first two days. After 10-days of heat a substantial proportion of 13 C-labeled pyruvate was allocated into de novo synthesis of VOCs. Thus, during extreme heat waves high respiratory losses and reduced assimilation can shift plants into a negative carbon balance. Still, plants enhanced their investment into de novo VOC synthesis despite associated metabolic CO 2 losses. We conclude that heat stress redirected the proportional flux of key metabolites into pathways of VOC biosynthesis most likely at the expense of reactions of plant primary metabolism, which might highlight their importance for stress protection.