With a solution ofp-aminoacetophenone, cerium(IV) gives a purple colour which can be used for its detection and determination. The limits of identification and dilution are 1.0 mug and 1 in 50000 respectively. Beer's law is obeyed in the range 14-350 mug. The colour is stable for only 5 min and measurements of absorbance must be made quickly. The interferences are fewer than with existing methods.