The extent to which exposure to new phonemic contrasts (i.e. contrasts that are present in the L2 but not in the L1) will lead to the creation of a new phonemic category in L2 speakers, as well as the phonological nature of these categories, remains an open question insofar as there is no consensus on whether acquiring a new contrast would result in abstract, phoneme-like categories, or if they belong to a less abstract level of representation. This work explores the perception of the /[open back unrounded vowel]/--/[open-mid back unrounded vowel]/ contrast (cop -- cup) in American English by Spanish speakers of L2 English through a discrimination task. The results show that while the interlanguage state of less experienced learners is best described as a case of single-category assimilation, the interlanguage state achieved by advanced learners is not a full phonemic split, despite the increased sensitivity to otherwise within-category perceptual cues; rather, it seems that while the ability to perceive differences is not affected, the ability to create a new phonemic representation is impaired.