The variation of rate with acidity is studied for the hydrogen exchange of 2-naphthol at the 1-position, for 6-amino-, 6-hydroxy-, and 6-methoxy-quinoline and for the 6-hydroxy-1-methylquinolinium cation at the 5-position, for 7-aminoquinoline at the 8-position, and for chromone and thiochromone at the 3-, 6-, and 8-positions. The exchange mechanisms are elucidated and the rates compared with literature data.