In this study, catalytic activity and deactivation mechanism of a carbon-based solid-acid catalyst in the esterification of oleic acid and methanol were investigated. The experimental results showed that the S150-4 catalyst, prepared by sulphonating at 150 degrees C for 4 h, yielded the highest sulfonic acid density. The conversion yield reached 97.98% under the optimum esterification conditions (i.e. catalyst loading, 10 wt %; methanol/oleic acid molar ratio, 8:1; reaction temperature, 65 degrees C; reaction time, 8 h) and decreased to 79.19% after four cycles. In the first batch of esterification, the formation of sulfonic esters was the main reason for the deactivation of the catalyst, whereas the leaching of sulphur was the main contributor in the second to fourth batches. Therefore, it is believed that reducing the leaching of sulphur and the formation of sulfonic esters by changing the addition method of methanol is an effective method to extend the service life of the catalyst. (C) 2020 Elsevier Ltd. All rights reserved.