The relative carbon isotope content (V13C value) in each position of glucose from a C4 plant (maize starch) and a C3 plant (sugar beet sucrose) has been determined by stepwise chemical and biochemical degradation of the molecule and stable isotope ratio measurement of the fragments. The suitability of the degradation methods has been tested through their chemical yield and isotope balance. The results from both methods agreed perfectly, revealing a defined and reproducible 13C distribution in glucose from both origins. Most prominent was a relative 13C enrichment by 5 to 6 a-units in position 4 and a depletion by about 5 s-units in carbon 6. As possible reasons for these nonstatistical isotope distributions, isotope effects of the aldolase, the triose phosphate isomerase, and the transketolase reactions during carbohydrate biosynthesis are discussed. The practical importance of the results in regard to isotope distributions in secondary plant products as a means for food authenticity control is outlined.

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