To avoid the problems caused by free fatty acids in the conversion of low cost vegetable oils to biodiesel, the use of solid acid catalyst for (trans-) esterification reaction is considered. Such a catalyst could be produced eco-friendly by using renewable raw materials such as biomass. The use of starch for this purpose it still very limited. In this paper, various methods were explored to produce a solid acid catalyst from corn starch. We investigated two different carbonization methods: complete pyrolysis in an oxygen-free environment and hydrothermal carbonization at milder conditions. Starch was used either in the native form or as pregelatinized starch. After the carbonization, acidic sites were introduced by sulfonating the materials. To characterize the catalysts, Scanning Electron Microscopy (SEM) was applied while the sulfonic content was determined by Energy Dispersive X-ray Spectroscopy (EDS). To test the performance of the catalysts, the conversion of free fatty acids was determined using oleic acid as a representative component of biodiesel feedstock. By both of the carbonization methods, a catalyst can be obtained that shows up to 84 % conversion of oleic acid. The hydrothermal treatment may then be preferred since it can be done at milder conditions. Differences between the performances of the respective catalyst samples could be well explained by structural features seen in the SEM-pictures. These also have their effect on the amount of sulfonic groups that was found (from EDS). The general trend is logical: the catalysts with a higher sulfonic load give a higher conversion of oleic acid.