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We have studied the magnetic and thermodynamic properties as well as the NMR spectra of the quasi two-dimensional Heisenberg antiferromagnet RbFe(MoO4)2. The observed temperature dependence of the order parameter, the critical indices and the overall magnetic H − T phase diagram are all in a good agreement with the theoretical predictions for a 2D XY model. The temperature dependence of the specific heat at low temperature demonstrates a crossover from a T 2 law characteristic of a two-dimensional antiferromagnet to a three-dimensional T 3 law.