The isothermal sintering behaviour of submicrometre-sized ( < 50 nm) powders of single-phase YBa2Cu3Ox (123) and unreacted stoichiometric mixture of submicrometre-sized ( < 50 nm) powders of BaCO3, Y2O3 and CuO (which on calcination at 1173 K gives YBa2Cu3Ox) was investigated through dilatometry under different sintering atmospheres. The sintering rate of the powder compacts was impeded by the presence of oxygen. The activation energies, Q, of sintering were determined to be 1218 kJ mol(-1) in argon, 1593 kJ mol(-1) in air and 2142 kJ mol(-1) in oxygen. A decrease in the apparent sintered density with increasing oxygen partial pressure was also observed. X-ray diffraction and thermal analyses (thermogravimetry and differential thermal analysis) showed no reaction during sintering of the single-phase product. Pellets fabricated from uncalcined powder exhibit two stages of sintering, one between 1073 and 1173 K having an activation energy Q=627 kJ mol(-1), and a second one above 1173 K with Q=383.7 kJ mol(-1). A.c. susceptibility, resistivity and critical current density were determined as a function of the temperature of the sintered samples.