The nature of the relationships between the level of immediately-prestimulus EEG activity and auditory ERP components remains unclear. Particularly, both inverse and direct relationships have been reported for the alpha band. Here we aim to clarify the pattern of prestimulus EEG contributions in alpha (8–13 Hz), and investigate those in beta (14–24 Hz), for five ERP components (P1, N1, P2, N2, P3) in an auditory equiprobable Go/NoGo paradigm. Separate FFTs were applied to the prestimulus Cz data of each accepted trial. The alpha and beta bands were independently assessed. The mean prestimulus spectral band amplitude was computed and used to sort the trials at nine central sites, and the upper and lower sorted trial thirds were averaged to form ERPs for Go and NoGo responses. Prestimulus EEG level effects (High vs. Low) were examined in each component's latency and amplitude, and Go reaction time was also assessed. Prestimulus alpha directly modulated the amplitude of the positive components (P1, P2, P3), while prestimulus beta directly modulated the positivity of the exogenous component amplitudes (P1, N1, P2); each amplitude effect occurred independently of the Go/NoGo stimulus conditions. Prestimulus beta also inversely modulated Go N1 latency; no reaction time effects were found for either band. The pattern of findings is intriguing and the various modulations are discussed in relation to attention and arousal. Together, these results confirm the importance of the EEG brain state immediately prestimulus, and indicate the considerable influence that these states have on event-related response processing.