The present invention relates to a brexpiprazole dihydrate crystalline form having a particle size distribution of d50 of at most 10 μm and less than 5% w/w of brexpiprazole anhydrate. The present invention also relates to a process for preparing this brexpiprazole dihydrate crystalline form and to pharmaceutical compositions thereof. The present invention also relates to a process for determining the absence of brexpiprazole anhydrate in this brexpiprazole dihydrate crystalline form. This brexpiprazole dihydrate crystalline form is used for the preparation of pharmaceutical compositions having delayed release properties upon intramuscular injection.