We have developed an analytical model to predict the onset of gel formation in ethylene/1-octene/1,9-decadiene terpolymerizations using constrained-geometry catalysts. The model relies on three kinetic parameters to characterize the catalyst response. Polymer resins have been synthesized in a continuous stirred-tank reactor to determine the model parameters, and to validate the model predictions for polymer properties and for the onset of gel formation and reactor fouling. The experimental results indicate that the free double bonds in 1,9-decadiene are as reactive as those found in 1-octene, and that the reactivity of 1,9-decadiene double bonds decreases after the 1,9-decadiene molecules become part of a polymer chain. The model predictions of polymer properties agree well with chromatographic, density, and mass-balance data. Moreover, the model was successful in preventing unintended reactor fouling during the duration of the experimental campaign. (C) 2009 American Institute of Chemical Engineers AIChE J, 56: 1325-1333, 2010