d13C values of 30 commercial juices were determined and their isotope ratio confronted to authentic lemon one’s. Among these 30 juices, 6 were elaborated from concentrate (ABC) and 24 were labelled as ‘‘pure juices’’. On their label, none of the 30 samples specified citric acid or sugars addition. d13C values of organic acids, reported on Table 2, are in the range 25.53‰ to 13.72‰. Some of these ratio are too high to be pure endogenous citric acid, i.e., an addition of ‘‘C4’’ type exogenous organic acids must have occurred. Glucose and fructose d13C values, for commercial juices, are in the range 27.56‰to 19.62‰. The higher d13C isotope ratio value reveals a sugar addition. These commercial samples results are plotted in Fig. 2 together with authentic lemons data illustrating the impact of ‘‘C4’’ organic acids or sugar addition on isotope ratios. 5 ‘‘ABC’’ and 5 ‘‘pure juices’’ present an organic acids d13C ratio much higher than expected and way outside the 95% confidence level. This means that for these 10 samples, the specification of organic acids addition was omitted on the label. In Fig. 2A, one sample has a d13C ratio higher than expected. Added ‘‘C4’’ type sucrose can be suspected. But, in Fig. 2B, fructose d13C ratio of this sample is comparable with authentic lemon fructose isotope ratios. Thus, this lemon juice must have been supplemented with glucose syrup, addition not mentioned on the label. In Fig. 2B, most of commercial juices d13C values are grouped and 10 of them are slightly outside the confidence level. Small addition of fructose syrup can be suspected. This suspicion is backed up with the similarity in the d13C vales of glucose and fructose (Table 2) which is usually not observed for authentic juices that present a gap of 1.8 ± 0.9‰. This fructose addition is not noticeable on a glucose/ fructose concentration ratio as the added amount is within the HPLC uncertainty (10%). This observation confirms the power of isotope measurement to detect ‘‘C4’’ type sugar addition even at low level. In Fig. 2, ‘‘pure juices’’ commercial samples supplemented with exogenous organic acids are highlighted with a dot in the symbols: it appears that among these 5 commercial samples, 2 of them seems to have been sugared as their fructose d13C ratios are slightly outside the confident level. A similar conclusion can be drawn for ABC juices as 2 of them are supplemented with organic acids and present a fructose d13C ratio slightly outside the expected range.