Alunite is a common sulphur-rich mineral in high-sulphidation epithermal deposits and can occasionally form independent alunite deposits, demonstrating a close spatial relationship with epithermal mineralization. In situ sulphur isotope analysis of alunite offers valuable insights into detailed mineralization processes. In this study, using LA-MC-ICP-MS, we evaluated matrix effects, spectral interferences and instrumental parameters that influence measurement accuracy, leading to the development of a high-precision microanalytical method for sulphur isotopes in alunite. In order to verify the results of the in situ method, LA-MCICP-MS measurements were compared with GS-IRMS analyses on two natural alunite samples: MT-1 and MTS-1. The internal precision of the in situ δ34S measurement was better than 0.22‰ when the 32S signal exceeded 5 V. The long-term reproducibility for these two samples was better than 0.33‰ (2SD). These results suggest that either of the two materials can serve as an in situ sulphur isotope standard. Based on GS-IRMS results, recommended δ34S values for MT-1 and MTS-1 are 9.37‰ ± 0.13‰ and 3.42‰ ± 0.31‰, respectively.