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[Display omitted] • The catalyzed mechanism of propargylic alcohol and H 2 O is investigated. • CO 2 could be absorbed by the [Bu 4 P+] cation to form [Bu 4 P -H -CO 2 ] complex. • A novel mechanism is proposed with the [Bu 4 P -H -CO 2 ][Im +H ] complex employed as the actual catalytic species. Hydration of propargylic alcohols is one of the most important pathways to synthesize α-hydroxy ketones. However, its mechanism is still obscure in the presence of multiple catalytic species. The mechanism of catalysis for the hydration of propargylic alcohols in the presence of [Bu 4 P+][Im-] is investigated along with the carbon dioxide (CO 2) as the co-catalyst by the density functional theory (DFT) with the aim to elucidate the role of [Bu 4 P+] cation and [Im-] anion. The CO 2 could be absorbed by the [Bu 4 P+] cation to form [Bu 4 P -H -CO 2 ] complex after the hydrogen atom in [Bu 4 P+] cation transfers to the [Im-] anion. Then, both [Bu 4 P -H -CO 2 ] and [Im +H ] are employed as the actual catalytic species to construct a new pathway. It is the more favorable as compared with the previously reported pathways. In previous reported pathway, the [Im-] anion is regarded as the real catalytic species, while [Bu 4 P+] cation plays the auxiliary role to stabilize the substrate by the hydrogen bond. The mechanism of the same reaction catalyzed by other four different ionic liquids is also investigated to further testify the contribution of [Bu 4 P+] cation and [Im-] anion in the catalysis. This work not only provides a new insight into the cation-regulated pathway for the hydration of propargylic alcohols but also points out a new strategy to design the ionic liquids achieving the improved catalytic efficiency. [ABSTRACT FROM AUTHOR]