Tall oil rosin was modified with fumaric acid or maleic acid anhydride to get fortified rosin, which was used as a raw material in fusion Ca-resinate syntheses. Two reaction steps were observed in the syntheses. A fast resination reaction takes place at 235 degreesC. A mechanistic model for the decarboxylation reaction step was derived to explain the effect of fortifying substances on the reaction kinetics. A multivariate calibration method using partial least-square (PLS) regression was applied to monitor the acid concentration decrease on the basis of infrared spectroscopic data during the fortified syntheses. A mixture of unmodified rosin and rosin oil (decarboxylated rosin) was used as a raw material in a novel modification of fusion syntheses to increase the mass yield of the resinate. In the modified rosin oil syntheses, decarboxylation of rosin acids is avoided, and recycling the added rosin oil considerably improves the mass yield of the resinate syntheses.