We present an experimental study of the dissociative recombination (DR) of H 3 O+ and its isotopomers D 3 O+ and HD 2 O+ performed at the ASTRID storage ring. DR cross sections have been measured as a function of energy, while complete branching ratios have been measured at E= 0. The H 3 O+ data yield an accurate determination of the branching ratio for water formation (0.25±0.01). The three molecular ions—H 3 O+, D 3 O+, and HD 2 O+—show a marked resemblance concerning cross sections and branching ratios. The only observed isotope effect is in the fragmentation pattern of HD 2 O+, where the release of a light fragment is favored over release of a heavier fragment. As a consequence, an enhanced production of deuterated molecules takes place as a result of the DR process.