This dissertation examines the process by which adult second language learners acquire phonological categories. It presents the results of experiments that examine the acquisition of vowel contrasts—specifically, the high front vowels /i/ and /

I

/ (as in English bead vs. bid), the high back vowels /u/ and /[special characters omitted]/ (as in boot vs. book), and the mid and low front lax vowels /ϵ/ and /æ/ (as in head vs. had)—by adult second language (L2) learners of American English with Russian backgrounds. The experiments focus on two aspects of the acquisition process: (1) the differential performance of L2 speakers on tasks that vary in terms of their proximity to natural speech conditions; and (2) the weighting of acoustic cues—specifically, vowel duration and spectral differences—in the categorization of these vowels. With respect to the first issue, the phonetic perception of vowel contrasts turns out to be better when the task concentrates speakers' attention on the contrasts and requires less involvement from the lexicon, both of which are conditions found primarily in the laboratory rather than in natural speech environments. With respect to the second issue, it appears that the L2 learners go through a stage of overreliance on duration in all three contrasts, but that the degree of overreliance is the greatest for the low vowel pair /ϵ-æ/ and least for the back vowel pair /u-[special characters omitted]/. Furthermore, the relative order of acquisition is found to be (1) /u-[special characters omitted]/, (2) /i-

I

/, and (3) /ϵ-æ/. The overall results do not provide support for the "desensitization" hypothesis (proposed by Bohn in 1995) suggesting that duration is the primary cue available to L2 speakers. The new data are discussed in relation to six theoretical models developed for first and second language acquisition of phonology.