For aqueous charge-stabilized polystyrene dispersions the ultrasonic sound velocity was observed to change with time. The drift could be as high as 17% per day. For certain systems, particularly concentrated dispersions at very low salt concentrations, sudden jumps in the sound velocity occurred. Dilute dispersions with added electrolyte were more stable, the variation being within the ppm range. By investigating different mechanisms (sedimentation, aggregation, ultrasonic streaming or coagulation) which might be responsible for the phenomenon, it was concluded that spontaneous ordering of the particles in the suspension is the cause of this drift. Ultrasound measurements appear to be more sensitive for the detection of the ordering process of the dispersions than optical investigation, since they sense the ordering of the particles well before the colloidal crystals become observable visually.