Dynamical response of water exposed to x-rays is of utmost importance in a wealth of science areas. We exposed isolated water isotopologues to short x-ray pulses from a free-electron laser and detected momenta of all produced ions in coincidence. By combining experimental results and theoretical modeling, we identify significant structural dynamics with characteristic isotope effects in H(2)O^(2+), D(2)O^(2+), and HDO^(2+), such as asymmetric bond elongation and bond-angle opening, leading …