The reversible magnetization Mrev of polycrystalline YBa2Cu3O7-δ (0<δ<0.5) was measured. Due to a small grain size a nd low critical current densities in oxygen deficient specimens Mrev could be measured over most of the superconducting regime from 10 K to Tc up to a magnetic field of 5.5 T. The magnetization data is in excellent agreement with calculated values obtained from Ginzburg-Landau (GL) theory. Values for the thermodynamic critical field Hc and the GL parameter κ are obtained. High values of κ ≈ 170-300 (for H∥c) are necessary to fit the magnetization data. Hc(T = 0) decreases strongly for increasing oxygen deficiencies up to δ ≈ 0.15, and then remains nearly constant. Specific heat jumps Δc at Tc were calculated from the data and the fit. Quantitative agreement with data in the literature is obtained. Just like Hc, the specific heat jump is strongly dependent on the oxygen deficiency in the near stoichiometric regime. © 1991.