Most previous studies related to loanword adaptation have centered on segmental mappings between source and loanword sounds in morphologically simplex words (LaCharité and Paradis, Ling Inq 36:223-258, 2005; Kang, Phonology 20:1-56, 2003). However, few have considered the adaptation of complex words, specifically words made of multiple free morphemes. Examining the adaptation of complex English words into Korean, the present study makes four claims. First, it proposes that each component of a complex word is a unit for loanword adaptation in calculating sound mappings. Second, it suggests that each component word is a stem, whereas a loanword as a whole is categorized as a nominal word in Korean. Third, apparent single-unit adaptation is possible only when the first component allows variable final vowel epenthesis at the end of the first component word; this is analyzed in terms of split-base effects. Fourth, the allophonic realization of phonemes plays a crucial role in loanword adaptation. These claims are empirically supported by loanwords found via the National Academy of Korean Language (NAKL 1991) and Google searching (March-June 2011). Furthermore, this study provides an explicit formalization of the analysis of complex loanwords within Optimality Theory (Prince and Smolensky, Opimality Theory: Constraint interaction in generative grammar, 1993). The present study contributes to the literature of loanword phonology by shedding light on several issues. First, the study proposes a model for the adaptation of complex loanwords wherein both morphological structure and the phonetic information of the source language play important roles. There has been intense debate about the effect of input information on loanword adaptation, broadly split between a phonological view (LaCharité and Paradis, Ling Inq 36:223-258, 2005) and a perceptual view (Silverman, Phonology 9:289-328, 1992; Steriade, in: Hume and Johnson (eds.) The role of speech