The oxygen nonstoichiometry, delta, has been determined for a spinel LiMn2O4-delta sample as a function of oxygen partial pressure pO(2) in the 873-1123 K temperature range, using thermogravimetric and chemical titration analysis. The maximum oxygen deficiency of LiMn2O4-delta was found to be delta(cr)=similar to 0.2. For the samples obtained at 1023 K, as delta increased from 0, the tetragonal spinel phase appeared around delta= 0.07, while only the cubic spinel phase was observed for the samples with delta less than or equal to 0.07. The dependence of delta on pO(2) was expressed as delta=similar to pO(2)((-0.67+/-0.06)). In order to explain this relation, we employed a defect cluster model consisting of four Mn3+ ions and two oxygen vacancies. On the other hand, the sample with delta=0.67 was found to decompose to LiMnO2 and Mn3O4.