The formation of pyridinium ions upon pyridine sorption on HY samples of varying Al composition has been studied by IR spectroscopy. It is demonstrated that HY supercages cannot accommodate more than four pyridinium ions. This property is shown to affect the ability of LF protons to migrate towards positions accessible to pyridine and form pyridinium ions. The mode of interaction of LF hydroxyls with pyridine is discussed in the light of both steric and electrostatic parameters.