The conglomeration of cerium oxalate crystals was investigated using ultrasound in a T-mixer and mixing tank reactor system. The mean particle size was enlarged when increasing the feeding time, yet insensitive to the length of the T-mixer. The use of ultrasound on the conglomerates revealed that they consisted of both aggregated and agglomerated particles. The aggregated particles were dispersed into individual particles with an ultrasonic intensity above 6 watts. However, the agglomerated particles resisted the vibrational force of the ultrasound up to 30 watts. Also, about 20 min of ultrasonic treatment was found to be sufficient to disperse the aggregated particles. The aggregation fraction in the conglomerates was estimated based on comparing the mean particle sizes obtained from the product suspensions with and without ultrasound treatment. By modifying the holding time in the stirred reactor and the conversion conditions in the T-mixer, the aggregation fraction was also found to depend on both the feeding time and the length of the T-mixer.