Two liquids, acetic acid and hexafluoroisopropanol (HFIP), and two solids, silica gel and polymethacrylic acid (PMA), were compared for hydrogen bond donor ability, acid strength, and catalytic activity in typical acid-catalyzed reactions, inversion of sugar and cleavage of acetone dimethyl ketal. In each pair, the weaker acid (HFIP and silica gel, respectively) was much the stronger hydrogen bond donor, but was totally devoid of catalytic activity, which the poor hydrogen bond donor but stronger acids (acetic and methacrylic acid, respectively) exhibited. A strong hydrogen bond donor (e.g., HFIP) enhances, however, the catalytic activity of the acid catalyst (AcOH). Thus, hydrogen bond donor ability is not a measure of acid strength. A correlation of the two properties is possible only when each group (acids and bases) involved in the comparison consists of very close structural relatives. Such a correlation cannot be extrapolated to any other case.