Abstract A series of metal-deficient spinels with compositions Cr 8(1−χ)/3In 8χ/3□ 1 3 S 4 exists for 0·4 ⩽ x ⩽ 1 at temperatures above 900°C and can be obtained at room temperature by quenching. The A sites contain In 3+ ions and vacancies in the proportion 2:1 independeny of x, and 2:1 ordering occurs for x = 1 whereas a random distribution is found for x = 0·833 and 0.667, and incomplete 1:1 ordering for x = 0·5 and 0·4. The mixed crystals segregate into In 2/3□ 1/3|In 1/3Cr 5/3|S 4 and nearly pure I □ 1/3|In 2| 4 (i.e. In 2S 3) at sufficiently low temperatures (below ⋍400°C). The experimental results cannot be understood on the basis of exclusively electrostatic interactions, and are discussed on the assumption that elastic energy plays an essential part.