Dehydration of multivalent ion-exchanged zeolites produced structural hydroxyl groups of varying thermal stability as evidenced by differences in the retention of their L-r. bands on heating in uacuo. The retention increased with the electron af6nity of the base-exchange cation. Although the i.-r. bands in the 3650 and 3545 cm-l regions appeared to have the same chemical identity as those formed by heating the NHZ-zeolite, these hydroxyl groups were less reactive with pyridine (Py). Temperatures of 85-150" were required to react a substantial fraction of them to PyH+, whereas with the decationated zeolites, both bands were removed by Py interaction at room temperature although the 3545 cm-' band could be restored to its original intensity by evacuation at 150".Py also co-ordinated with the base-exchange cations (became Lewis bonded). The strength of this interaction (retention of PyL at elevated temperatures) increased with the electron affinity of the cation and the frequency of the corresponding i.-r. band increased concomitantly from 1443 cm-' (Na+) to 1455 cm-l (Zn2+). Addition of H20, or H20+C02, effected conversion of PyL to PyH+, and again the extent of these reactions paralleled the electron affinity of the cation.