We present an accurate investigation of the prototypical rare-earth cobaltite GdBaCo 2 O 5.0 by complementary synchrotron powder and conventional source single-crystal x-ray diffraction experiments. The correct space group (Pmmm) and the accurate crystallographic structure of this compound at room temperature (RT) have been determined. By increasing T a second-order structural-phase transition to a tetragonal structure with space group P4/mmm at T ≈ 331 K is found. Close to the Néel temperature

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... (T N ≈ 350 K), anomalies appear in the trend of the lattice constants, suggesting that the structural phase transition is incipient at T N . A possible mechanism for this complex behavior is suggested.