We present an electrochemical study of BiSbO4, an opened layered oxide having a structure related to Aurivillius phases. Li//BiSbO4 cells show a large specific capacity as high as 1250mAhg−1 during reduction down to 0.5V. This reaction involves 18Li atoms per formula unit, pointing it towards a very promising cathode material for primary lithium batteries, in particular for ICD devices. The characterization of the reduction products indicates that the reduction of BiSbO4 with lithium presumably goes along firstly with the formation of metallic Sb and Bi to follow the formation of the alloys Li3Bi and Li3Sb dispersed in a lithium oxide matrix. In situ X-ray diffraction experiments proved the amorphous nature of both metals and final alloys. On the other hand when Li//BiSbO4 cells are limited to discharge down to 1.2V, BiSbO4 reacts with 5Li atoms. After the first discharge, that develops a specific capacity of 350mAhg−1, high cyclability has been observed.