The specific heat of porous YBa2Cu3O≈7.00 ceramics with two well separated steps at the superconducting transition was measured in magnetic fields up to 14 T. The same measurements were performed for an YBa2Cu3O≈6.93 ceramics from a similar batch annealed reversibly to obtain one single sharp transition. The results show that both components of the double transition, as well as the single transition, are transformed in the same way under the application of a field, both in position and width. It is concluded that both components of the split transition are of superconducting origin, and are related with the existence of two kinds of homogeneous domains, the typical sizes of which are large compared to the superconducting coherence length. Depending on the criterion chosen for the definition of Tc (H), the angle-averaged critical-field slopes μ0∂Hc2/∂T|Tc are found to range from −5.4 to − 10.1 T/K for all transitions. The slopes for both components of the split transition are equal within ±10%.