Ultrasonic technology can be used to monitor the crystallization of fats and determine solid fat content (SFC) online. Ultrasonic waves are attenuated as crystals form and grow, and this attenuation occurs first at higher frequencies. The attenuation of the ultrasonic signal does not depend on the induction times of crystallization of the systems, or on their thermal behavior; but it does depend on SFC and on microstructure, particularly on the crystal size. At low SFC values (≈5%), bigger crystals generate more attenuation. At intermediate SFC values (≈10%), crystal size does not affect signal attenuation and SFC is the key factor responsible for signal attenuation. At high SFC values (up to 20%), crystal size again seems to be the factor that controls attenuation.