Abstract: Executive dysfunctions (regarding behavioural inhibition, decision making, flexibility or voluntary selection) rank among the core symptoms of attention deficit/hyperactivity disorder. Several studies demonstrated functional variations in patients with ADHD especially during response inhibition and flexibility. However, information about functional correlates of other aspects of executive functions such as voluntary selection processes is limited. A group of thirty adult patients with attention deficit/hyperactivity disorder (ADHD) and 30 healthy controls, matched for age and education, participated in the present study. Electrophysiological responses (event-related potentials, gamma oscillations) and behavioural data were acquired during the voluntary selection between various response alternatives. ADHD patients demonstrated increased responses in the gamma frequency band especially in frontal and fronto-central brain areas during voluntary response selection processes compared to healthy subjects. In addition, the error rate was increased in patients. Given that gamma-band responses have been related to GABAergic and glutamatergic responses these results may indicate accordant dysfunction in patients with ADHD.