The activation of 1-siloxy-substituted zirconocene with MAO occurs slowly, in 4-6 h. The slow activation process involves three distinct ligand to metal charge transfer (LMCT) band shifts in the UV/vis spectrum before the maximum LMCT absorption energy and intensity of the third appearing band is reached. In addition, at very high MAO concentrations, a fourth band at 460 nm begins to form. On the contrary, the corresponding 2-siloxy-substituted as well as the unsubstituted zirconocene react with MAO in less than a minute, immediately producing the final species, as confirmed by only one observed LMCT band shift. The 1-silyl-substituted zirconocene reacts with MAO, producing the final active species in 1 h. UV/vis spectroscopic observations were interpreted with the aid of molecular modeling studies, demonstrating that the interactions of the oxygen donor atom of the siloxy substituent with electron deficient Zr center and coordinatively unsaturated Al in MAO play a prominent role in the activation process.