The S-1-S-0 electronic spectra of 3,7-dichlorotropolone, 3,5,7-trichlorotropolone, and 3,7-dibromotropolone have been measured in a supersonic free jet to investigate the effect on proton tunneling of symmetrical substitution of Cl or Br atoms for H atoms of tropolone. The 0(0)(0) tunneling splittings in S-1 of 3,7-dichlorotropolone, 3,5,7-trichlorotropolone, and 3,7-dibromotropolone are 45, 31, and less than or similar to cm(-1), respectively. The increase of the tunneling splitting for 3,7-dichlorotropolone compared to that for tropolone (19 cm(-1)) is attributed to a decrease in the O-H...O heavy atom separation, leading to smaller potential barrier height. The decrease of the 0(0)(0) tunneling splitting for 3,5,7-trichlorotropolone compared to 3,7-dichlorotropolone and remarkable decrease of the 0(0)(0) tunneling splitting for 3,7-dibromotropolone are ascribed to significant changes of the normal coordinates compared to those of tropolone.