A homogeneous batch of YBa2Cu3Ox porous ceramics was studied in detail as a function of the annealing temperature Ta. X-ray patterns and optical micrographs show definitely one single phase. Well separated double transitions are however observed, most clearly in the specific heat curves, except for Ta≈460°C. Initial values are Tc1=87 K and Tc2=92 K for Ta=300°C. Upon increasing the annealing temperature to Ta=500°C, Tc1 increases to 93.5 K while Tc2 remains almost constant. Beyond Ta=500°C, both Tc1 and Tc2 decrease. The reversible splitting of superconducting transition appears to be due to the complex kinetics of oxygen uptake. Tc1 is attributed to the external layer of the grain and Tc2 to the grain cores.