Seven ortho rotational transitions have been recorded for the main isotopic species of water in the millimeter- and submillimeter-wave region using the Lamb-dip technique in order to resolve the hyperfine structure due to the hydrogens and to provide accurate hyperfine constants. The experimental determination has been supplemented by high-level quantum-chemical calculations of the hyperfine parameters thereby focusing in particular on a systematic study of the basis-set convergence and on vibrational effects. (C) 2009 Elsevier B. V. All rights reserved.