Abstract: This research is directed at characterizing purified maleopimaric acid (MPA), looking at the amount of MPA and fumaropimaric acid (FPA) made from a large amount of rosin and maleic anhydride (MA) or fumaric acid (FA) with various molar ratios and sources of rosin and evaluating the properties of fortified rosin sizes made from both MPA and FPA. Results showed that identification of the Diels-Alder adduct of abietic acid and maleic anhydride using Mass spectrometry produced a mixture of endo-maleopimaric acid methyl ester with endo-maleopimaric acid tri methyl ester, as indicated by a fragment ion at m/z = 146 with a molecular weight of 414 and fragment ions at m/z = 121, 187, 316 and 386, denoting an endo-maleopimaric acid methyl ester. A fragment ion at m/z = 146 with a molecular weight of 460 and other fragment ions at m/z = 187, 121, 400 and 428 indicated endo-maleopimaric tri methyl ester. Using a large amount of rosin as a raw material to produce MPA, the equation Y = -0.8475 X2 + 10.448X - 9.7125, at a reaction temperature of 200 °C is still relevant as it denoted that a molar ratio of 1:6.2 (rosin and MA) is the best. However, the equation Y = -0.46X2 + 5.268X - 4.47 did not apply to FPA. Using a large amount of rosin, an increase in the molar ratio led to an increase in FPA products. In terms of free rosin and pH, the maleo-and fumaro-pimaric rosin sizes have met the requirement of Indonesian national standards for paste rosin size. In terms of free alkali property, the maleo- and fumaro-pimaric rosin sizes were better than the free alkali of the commercial forms.