NMR, muon spin rotation (SR), 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 Sϭ1/2 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 ϭ1.1Ϯ0.1 with J 2 ϩJ 1 ϭ8.2Ϯ1 K). At T c Ӎ2.8 K a phase transition to a low-temperature collinear order is observed. T c and the sublattice magnetization, derived from NMR and SR, were found practically independent on the magnetic field intensity up to 9 T. The critical exponent of the sublattice magnetization was estimated ␤Ӎ0.235, 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 .

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