The instantaneous amplitude of the theta and alpha bands of the electroencephalogram (EEG) was studied during preparation periods in a task-switching experiment. Subjects had to switch between tasks in which they were to respond to either the visual or the auditory component of the stimulus. 11-13 Hz occipital amplitude increased prior to auditory, relative to visual repetition trials. The effect was transient, ending well before presentation of the stimulus that was being prepared for. Alternation trials were preceded by an increase in occipital theta-band activity, relative to repetition trials, for the visual task. This effect was also transient. The effects suggest tentative hypotheses for the function of transient bursts of alpha- and theta-band oscillations and indicate the possibility of a psychophysiological resolution of theoretical questions concerning the origin of switch costs.