  NMR, muSR, magnetization and specific heat measurements in Li(2)VOSiO(4)
powders and single crystals are reported. Specific heat and magnetization
measurements evidence that Li(2)VOSiO(4) is a frustrated two-dimensional S=1/2
Heisenberg antiferromagnet on a square lattice with a superexchange coupling
J_1, along the sides of the square, almost equal to J_2, the one along the
diagonal ($J_2/J_1=1.1\pm 0.1$ with $J_2 + J_1 = 8.2\pm 1$ K). At $T_c\simeq
2.8$ K a phase transition to a low temperature collinear order is observed. T_c
and the sublattice magnetization, derived from NMR and muSR, were found
practically independent on the magnetic field intensity up to 9 Tesla. The
critical exponent of the sublattice magnetization was estimated $\beta\simeq
0.235$, nearly coincident with the one predicted for a two-dimensional XY
system on a finite size. The different magnetic properties found above and
below T_c are associated with the modifications in the spin hamiltonian arising
from a structural distortion occurring just above T_c.
