The flexural strengths of rectangular YBa2Cu3O6+δ bars, prepared from mixed oxides and carbonates or spray-dried precursors, have been measured at room temperature and at 77 K. Strengths ranged from 17.8 to 57.6 M Pa at room temperature, depending on processing history, and were 20% greater when measured at 77 K. Corrosion of YBa2Cu3O6+δ in humid air at 38° C created two layers of corrosion products, but did not weaken the uncorroded core when failure loads were corrected for the decreased sample dimensions. The Knoop hardness of polycrystalline YBa2Cu3O6+δ ranged from 436 to 447 KHN while the hardness of individual grains of YBa2Cu3O6+δ was 498 KHN. Variations in flexural strength with microstructure were observed and are discussed.