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The inflorescences of Cannabis sativa L. represent a high-value by-product of industrial hemp cultivation, and their volatile fraction has recently attracted attention due to the applications of hemp-derived Essential Oil (EO) in several fields. Following a previous evaluation of the EOs profile in diverse hemp genotypes, the current study investigated how the phenological stage of inflorescences affects the EO chemical composition and hydrodistillation yield. Two hemp varieties with different flower behaviour, Codimono (monoecious) and Fibrante (dioecious), were cultivated in open field, and the flowering tops were collected at five developmental stages. The air-dried samples were hydrodistilled with a Clevenger apparatus, and the obtained EOs were analysed by Gas Chromatography coupled with Mass Spectrometry (GC-MS). All the samples showed terpenes and phytocannabinoids as major chemical classes. Among the former class, sesquiterpene hydrocarbons were the most abundant components in the EOs at all phenological stages, ranging from 34.1% to 67.6% and 33.8–73.8% in Codimono and Fibrante, respectively, while oxygenated sesquiterpenes were highest at the final stage (27.7% in Codimono and 24.0% in Fibrante). Phytocannabinoids, instead, were the second major chemical class, accounting for 29.3% in Codimono and 30.5% Fibrante. The EO yield increased with phenological development, reaching the highest rate at seed harvest time in both genotypes (0.018–0.265% w/w in Codimono and 0.026–0.265% w/w in Fibrante). [Display omitted] • Phytocannannabinoids are greatest at seed maturity in inflorescences and their EO. • Sesquiterpene hydrocarbons are the main chemical class of the hemp essential oil. • BBCH 69 and 87 are the optimal stages for the EO production. [ABSTRACT FROM AUTHOR]