The invention relates to a process for the cost-effective and environmentally responsible preparation of alkyl indium sesquichloride in high yield and with high selectivity and purity. The alkyl indium sesquichloride prepared in accordance with the invention is particularly suitable, also as a result of its high purity and yield, for preparation of indium-containing precursors in accordance with demand, in high yield and with high selectivity and purity. As a result of the high purity, the indium-containing precursors that are preparable are particularly suitable for metal-organic chemical vapor deposition (MOCVD) or metal-organic vapor phase epitaxy (MOVPE). The novel process according to the invention is characterized by the improved execution of the process, in particular by rapid process control. Owing to targeted and extensive use of raw materials that are inexpensive and have a low level of environmental pollution, the process is also suitable for use on an industrial scale.