Kinetics reaction between castor oil and maleic anhydride without condensation and cat- alyst was studied. On the other hand, the products concentration, kinetics and thermodynamics para- meters were determined using mathematical model- ing and were validated with experimental data. The reaction was carried out in a semi-batch reactor at constant temperature. The reaction was conducted through the determining of the acid number (ASTM D4662-03), and showed first order kinetics with re- spect to the anhydride. For the modified oil, the hy- droxyl value (OHv) was quantified through titration following (ASTM D4274-05). The distribution of mo- lecular weights was determined using gel permeation chromatography (GPC), and functional group analy- sis was determined using infrared spectroscopy (IR).