BACKGROUND: The increasing value of pure vegetable oils has made the use of waste cooking oils an attractive alternative feedstock for biodiesel production but the presence of free fatty acids in the waste significantly reduces efficiency of the conversion. RESULTS: A low-temperature mixed ionic liquid system with dual extraction and catalytic functions for the conversion of waste oils containinghigh levels of free fatty acids to biodieselwas successfully trialled. The solvent, a totally miscible system ofHPyrBr and the Bronsted acid, SPyrHSO(4), allows the key properties of each ionic liquid, as extractant or catalyst, towork uninhibited by the presence of the other. Extraction of free fatty acids from vegetable oils and their conversion to methyl esters in high yield is demonstrated for vegetable oils, containing single and mixed fatty acids, and for a model waste cooking oil. The oil separated from free fatty acids in the waste has a low acid value and can be converted efficiently to biodiesel; with the ionic liquid solvent recovered unchanged for reuse. CONCLUSIONS: A mixed ionic liquid extraction-esterification solvent process has been developed to effect efficient conversion of free fatty acids in waste vegetable oils to biodiesel and their conversion to esters. (C) 2017 Society of Chemical Industry