['Following external stimulation, electroencephalographic (EEG) responses from different frequency bands occur simultaneously, but little is known about whether and how concurrent multi-frequency responses depend on each other during stimulus information processing. The present study assessed the effects of task stimulus relevance on locally co-existent time-frequency components of event-related brain potentials (ERPs).', 'The wavelet entropy (WE) of ERPs was used as an analytical tool because low entropy values correspond to a narrow-band (mono-frequency) activity characterizing highly ordered (regularized) bioelectric states. The minimum of WE in the ERPs (WEmin) was identified to reflect a transient dominance of one particular frequency ERP component over other frequency components. In an auditory oddball condition, effects of stimulus relevance were analyzed for the timing, rate of decrease, and frequency determinants of WEmin in 10 subjects.', 'Major results demonstrate that a highly ordered EEG microstate emerged in response to both target and non-target stimuli, as evidenced by the substantial decrement of ERP entropy. This microstate (1) was short lasting as indexed by the transitory entropy decrease, (2) had a functionally specific time-localization as reflected by stimulus and electrode effects on WEmin latency, and (3) for both stimulus types was determined by a pronounced dominance of locally synchronized theta (4-8 Hz) oscillations.'] Does a transient dominance of theta event-related brain potential component characterize stimulus processing in an auditory oddball task?