A method of operating hydrocyclone comprising a separation chamber which in use is arranged to generate an internal air core for affecting a material separation process, comprises measuring both a vibrational parameter of the separation chamber and a stability parameter of the internal air core during operation of the hydrocyclone. The measurements are compared against predefined corresponding parameters which are indicative of a stable operation of the hydrocyclone and an operational parameter of the hydrocyclone is adjusted dependent on the comparison.