Disentangling the roles of phonological well-formedness and lexical attestedness in phonotactic processing has proven challenging. In this study, we present results from a passive listening ERP study showing that English speakers exhibit distinct neural responses to CCVC nonce words according to the phonological well-formedness and attestedness (in English) of the onset cluster. Clusters with poor sonority sequencing evoked an N400 effect compared to those without poor sonority sequencing, regardless of whether the well-formed clusters were attested in English. In contrast, unattested clusters, regardless of whether they were well-formed or ill-formed in terms of sonority sequencing, evoked a late positivity compared to attested clusters. The results suggest that listeners first perform a phonological analysis on potential words before submitting them to a lexical search.