The reactivity of the metallocarbohedrene cluster Ti8C12+ is studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Rate constants for the addition of various polar molecules (NH3, H2O, and CH3CN) and nonpolar molecules (C2H4 and C6H6) to Ti8C12+ are determined by isolating each Ti8C12(L)(x)(+) (L = NH3, H2O, CH3CN, C6H6, or C2H4) species and monitoring the decay of the parent ion as a function of time. For NH3 and H2O, four fast additions are observed, with additional associations occurring at a greatly reduced rate, while reactions of Ti8C12(CH3CN)(x)(+) (x = 0-4) with CH3CN come to a complete halt after the association of the fourth molecule of CH3CN. We propose that ligand polarity plays a key role in the dramatic decrease in rate constant observed for addition reactions which occur after the attachment of a fourth polar molecule to the cluster.