Models such as Eckman's markedness differential hypothesis, Flege's speech learning model, and Brown's feature-based theory of perception seek to explain and predict the relative difficulty second language (L2) learners face when acquiring new or similar sounds. In this paper, we test their predictive adequacy as concerns native English speakers' mastery of French /Â/ and Spanish /|/. Based on an acoustic analysis of the learner data, we demonstrate that these three models do not account for the full range of variability nor for the developmental sequences attested, because they do not consider the degree of difficulty involved in the simultaneous mastery of multiple phonetic parameters across prosodic positions. Consequently, models of L2 phonological acquisition must not only integrate findings from markedness theory and speech perception but also incorporate phonetic constraints on production.One of the central goals of research on second language (L2) phonological acquisition is to model interlanguage development. This includes not only describing but importantly also explaining and predicting the relative difficulty learners of a given first language face when acquiring segments absent from or similar to those of their first language (L1). To date, most research in this vein has focused on intralinguistic difficulty, that is, the relative challenge of acquiring two or more new phonological structures in a given target language (TL), particularly sounds involved in an opposition (e.g., English /l/ and /®/, e.g., Larson-Hall, 2004; voiceless vs. voiced coda obstruents, e.g., Hancin-Bhatt, 2000). These accounts have made their predictions based principally on typological markedness (