Up to 50 °C, sotolon 1 slightly decomposed to acetoin 5 in aqueous sodium hydroxide solution under nitrogen and in the absence of light, and this degradation was irreversible, but at 80 °C it was rapidly decomposed according to two other reaction pathways. The first one (kinetic route) was new: it gave rise to the novel 3-methyl-2-oxopent-3-enoic acid 11 by opening of the lactone cycle. The second one (thermodynamic route) was the reverse of the formation of 1 from ethanal 9 and 2-oxobutyric acid 8, giving also propionic acid 10 by decarbonylation of 8. As shown in dilute potassium hydroxide added with potassium fluoride, the initial addition of hydroxide ion to 1 distinguished between the processes at 50 and 80 °C. At 50 °C, the ion added to the soft electrophilic carbon of the conjugated lactone (1,4-addition), whereas at 80 °C, it added to the hard electrophilic carbonyl carbon (1,2-addition). Under acidic conditions, 11 was easily cyclised back to racemic 1, whereas 1 formation from ethanal and 2-oxobutyric acid was very slow and unspecific, as reported previously. Furthermore, 5,5,5[2H3]-3-methyl-2-oxopent-3-enoic acid d3-11 was produced from 1, when its degradation was carried out in sodium deuteroxide at 80 °C. Its cyclisation in deuterated acid solution gave rise to 3-hydroxy-4-methyl-5-[2H3])-methylfuran-2(5H)-one d3-1, which was shown to be chemically and isotopically stable in strong acidic conditions. d3-11 was used as internal standard in a new stable isotope dilution assay method to quantitate 1 in wine. As 1 was shown to decompose slowly to the above degradation products in dilute sodium carbonate at room temperature, but faster in the presence of cupric ion (mainly oxidation to 5), a purification procedure using the favoured partition of hydrophilic 1 in dilute aqueous acid allowed to obtain extracts suitable to quantitate specifically 1. This method allowed the quantitative determination of 1 in Petit and Gros Manseng dry and sweet Jurançon wines (9–24 years ageing), with levels below its olfactive perception threshold (15 μg L−1).