Tomato passata, one of the ‘pillars’ of the Mediterranean diet, is a typical Italian product that by law must be obtained exclusively from fresh tomatoes. For this reason Italian law provides for use of the δ 18O of vegetal water to determine whether passata (Brix of up to 12) is genuine or has been obtained by diluting tomato paste (Brix higher than 12), although without any indication of reference limits. In this study an extensive dataset of over 1000 samples collected along the Italian tomato product production chain (tap waters, juices, passata, pastes) in different years (2004-2012), was investigated on the content of vegetal water δ18O. This parameter was shown to be statistically different in the four classes of products, increasing from water to juice and paste, in relation to the different degree of evaporation taking place during the preparation phase. Moreover, the δ18O of vegetal water, due to the contribution of the lower δ18O values of tap water during dilution, was highly effective in distinguishing genuine passata from watered down paste, also in the case of passata previously concentrated up to 11.9 Brix. In particular, two lower threshold values are proposed: -1.4‰ for ‘raw’ passata and -2.9‰ for passata previously concentrated up to 11.9 Brix. Although the use of δ18O water alone does not make it possible to discriminate between Italian and foreign products, an increasing trend in δ18O water values was shown in juices and passata samples as well as in tap waters, when moving from northern to southern Italian products, making it possible to predict good geographical discrimination for this parameter.