Esterification of glycerol with acetic acid was studied using a series of Brønsted acidic ionic liquids as catalysts. The results indicate that double SO₃H-functionalized ionic liquids show high catalytic activity and fair reusability even at very low catalyst loadings, while the conventional non-functionalized ionic liquids show poor activity. The Brønsted acidity–catalytic activity relationships were also investigated and the results showed that the sequence of the catalytic activity observed in the transformation was in good agreement with the Brønsted acidity order determined by the Hammett method.