We have investigated the nuclear spin conversion of H2O in solid argon. At 4.2 K and for high dilution, we have measured a limit time of 680 +/- 30 min. We have not observed any change of the conversion time upon double doping with CO2, CO, N-2 and O-2. No effect could be detected due either to the spectrometer light, the golden mirror surface or the crystalline quality of the matrix. Therefore, we can conclude that this time is only due to the interaction with the matrix, which may enhance nuclear spin conversion induced by intramolecular spin-rotation coupling. (C) 2007 Elsevier B.V. All rights reserved.