Zero and longitudinal field muSR measurements in Pb2VO(PO4)2 and BaCdVO(PO4)2, two prototypes of the frustrated S=1/2 square lattice model with competing ferromagnetic and antiferromagnetic interactions, are presented. Both systems are observed to undergo a phase transition to a long-range magnetic order at TN~3.46 K, for Pb2VO(PO4)2, and at TN~0.99 K, for BaCdVO(PO4)2. In Pb2VO(PO4)2 both the temperature dependence of the order parameter and the longitudinal relaxation rate above TN are consistent with a twodimensional XY model, as it is found for Sr2CuO2Cl2. On the other hand, for BaCdVO(PO4)2, which lies very close to the magnetically disordered region of the phase diagram where a bond-nematic order was predicted, a peculiar logarithmic increase in the relaxation is observed above TN. In both systems a rather broad distribution of internal fields at the muon sites is noticed below TN. The origin of this distribution is discussed in the light of the muSR experiments already performed on S=1/2 frustrated antiferromagnets on a square lattice.
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