We report the first observation of five water trimer isotopomers using terahertz laser spectroscopy-coupled with a pulsed slit jet expansion technique. A single c-type vibration-rotation-tunneling (VRT) band has been observed for each isotopomer between 40 and 50 cm(-1). By considering all the experimental data, including results from isotopic substitution experiments, analyses of bifurcation tunneling splittings, and rotational analyses of the VRT bands, it has been possible to determine unambiguously the isotopic composition and structure of each isotopomer. We have also extended the measurements of the 41.1 cm(-1) band in (D2O)(3), first reported by Suzuki and Blake [Suzuki, S.; Blake, G. A. Chem. Phys. Lett. 1994, 229, 499]. All six VRT bands have been assigned to pseudorotational transitions, which principally involve flipping of the ''free'' hydrogen or deuterium atoms in the trimer from above to below the plane of the OOO ring. Interestingly, four of the six bands have been assigned to the same transition. The four trimers responsible for these bands, designated d(6), d(5)a, d(4)a, and d(3)a, form a series in which the three deuterium atoms in the ring are sequentially substituted by hydrogen atoms. The corresponding experimental band origins for d(6) to d(3)a show a 1-2 cm(-1) blue shift upon each substitution. Existing pseudorotation models are unable to satisfactorily account for this result. We propose that, if the hydrogen or deuterium atoms within the ring are allowed to participate in the flipping motion, this band origin shift can be explained.