The viscosity of carboxylic‐acid‐terminated polybutadiene increases markedly upon neutralization with mono‐ and divalent bases. These viscosity increases are related to apparent molecular weight, considering the association of the terminal salts as ``polymerizing'' the difunctional prepolymer. For the monovalent alkali‐metal salts, simple end‐group dimerization seems to occur with an average &Dgr;Hof ∼−25 kcal/mole. Neutralization with ions of higher valency leads to materials with higher viscosities.