The present study aimed to investigate neurophysiological substrates of phoneme and word processing in 10 patients with acute aphasia (PWA). More specifically, phoneme discrimination was studied in a passive and active oddball task with respect to different phonemic contrasts, while lexical detection was investigated by presenting infrequent pseudowords among frequent words in a passive oddball task. Concerning phoneme discrimination, PWA in the acute stage had smaller MMN and P300 amplitudes than the norm group for voicing, whereas for place and manner they only demonstrated smaller P300 amplitudes. PWA showed a distinct pattern of impaired phonemic contrast sensitivity, with place displaying the largest amplitude and voicing the smallest. Concerning lexical detection, pseudowords elicited larger responses than words in both groups, but with a delay and larger P200 amplitude for pseudowords in PWA compared to the norm group. For clinical practice, passive tasks seem more suitable than active tasks in acute aphasia.