Single water molecules have been isolated in solid Ar matrices at 4 K and studied by rovibrational spectroscopy using FTIR in the regions of the v(1), v(2), and v(3) modes. Upon nuclear spin conversion at 4K, essentially pure para-H(2)O was prepared, followed by subsequent fast annealing generating ice particles. FTIR studies of the vapor above the condensed water upon annealing to T >= 250 K indicate fast reconversion of nuclear spin to equilibrium conditions. Our results indicate that nuclear spin conversion is fast in water dimers and larger clusters, which preclude preparation of concentrated samples of para-H(2)O, such as in ice or vapor.