ABSTRACTWe have investigated nuclear spin symmetry conservation and relaxation of water seeded in argon or molecular oxygen using supersonic jet expansions probed by high-resolution continuous-wave laser cavity ring-down spectroscopy. The probing of the R-branch of the 2 band (above 7500 cm1) was used to complement our previous investigation. We were able to further cool down the water samples (down to 17 K., i.e. 7 K colder than before) and double the data set (regarding argon as carrier gas). We confirm our first observations: at the lowest rotational temperatures and low partial pressures of HO, nuclear spin symmetry is conserved, in agreement with theoretical expectation for inelastic collisions. For high concentrations of water in the gas mixture, we obtained higher rotational temperatures and were able to observe nuclear spin symmetry relaxation. This can be related to the formation of water clusters at the early stage of the supersonic jet expansion. We also present the analogous series of measurements performed using molecular oxygen (O) as carrier gas: we observed similar behaviour for low and high concentrations of water in the gas mixtures when cooling the samples to 16 K. The conservation of nuclear spin symmetry at low temperature and low concentration using oxygen as carrier gas indicates that the paramagnetic collisional partner does not play a significant role regarding the possible nuclear spin symmetry conversion of water. We discuss possible mechanisms related to our observations of apparent nuclear spin symmetry relaxation.