This study introduces a new nanocrystallization method assisted by ultrasounds that produces needles or nanocubes of CMONS, a stilbene dye, with excellent control over polymorphism and narrow size distribution. Owing to the production of radicals from dissolved dioxygen by high-intensity ultrasounds, trans-to-cis isomerization was observed in the absence of nitrogen bubbling, with the formation of two distinct crystalline phases for different diastereomers. (400)