Recent studies have suggested that proficient bilinguals show morphological decomposition in the L2, but the question remains as to whether this process is modulated by the cognateness of the morphemic constituents of L2 words and by L2 proficiency. To answer this question was the main goal of the present research. For that purpose, a masked priming lexical decision task was conducted manipulating for the first time the degree of orthographic overlap of the L2 word as a whole, as well as of their morphemic constituents (bases and suffixes). Thirty-four European Portuguese-English bilinguals (16 intermediate and 18 high-proficient) and 16 English native-speaking controls performed the task in English. Results revealed that both groups of bilinguals decomposed words as the native control group. Importantly, results also showed that morphological priming effects were sensitive not only to cross-language similarities of words as a whole, but also to their morphemic constituents (especially, suffixes).