The aim of this work was to investigate the effects of different isolation methods on the chemical composition of the essential oils of rock samphire (Crithmum maritimum L.) and composition of the volatile compounds in the hydrolates, that remain as a by-products of essential oil isolation. Conventional distillation methods, water distillation (HD) and steam distillation (SD) were used, as well as a modern "green" method, microwave-assisted distillation (MHD). Isolation of volatile compounds from the hydrolates was performed by solid phase microextraction (HS-SPME). A coupled system of gas chromatography with mass spectrometry (GC-MS) was used to separate and identify the chemical components of the essential oils and hydrolates. The essential oils obtained by SD had higher levels of terpinene-4-ol, γ-terpinene and limonene than those obtained by HD and MHD. Sabinene was most abundant in the MHD essential oil, while E-(β)-ocimene was most abundant in the essential oil obtained by HD. In the HD hydrolate, 10-(acetylmethyl)- 3-carene and benzacetaldehyde dominated, while terpinen-4-ol was present with the highest proportion in the MHD hydrolate. In the SD hydrolate, terpinen-4-ol, (Z)-sabinene hydrate and (E)-sabinene hydrate stand out. The chemical composition of hydrolates differs from that of essential oil, both in terms of the type and number of different compounds that are absent in essential oils. From this study, it can be concluded that the isolation method has a great influence on the profile of rock samphire essential oils, and especially on the profile of compounds from hydrolates. Due to its valuable chemical composition, these by-products are valuable source of bioactive compounds with great potential for use in various industries.