Abstract Oxygen nonstoichiometry in Bi 2Sr 2CaCu 2O 8+δ has been investigated using a number of thermogravimetric and titrimetric techniques. Apparent discrepancies between the results obtained by different methods have been resolved and can be explained by consideration of the chemical processes involved in the analytical procedures. For samples prepared in air at 850°C and air quenched the total oxygen constant is 8.19 O per formula unit. Titrimetric measurements show that there are two components to the oxygen excess in Bi 2Sr 2CaCu 2O 8+δ. One, 0.14 O, leads to oxygen evolution on dissolution; the other, 0.05 O, retains its high oxidation state on dissolution. Thermogravimetric measurements also show that the oxygen excess consists of these two components. One part, 0.06 O per formula unit, can be removed by heating in air at temperatures just below melting and is easily reduced by hydrogen. The remainder, 0.13 O, is more difficult to remove, persisting after most of the bismuth and copper oxygens have been removed by hydrogen.