This study examined the relationship between learners' motivation to learn a second language (L2) and persistence in their learning using self-instructional radio (SIR) materials with a sample of Japanese high school students learning English. L2 self-instruction remains under-researched, in spite of the importance of out-of-class learning emphasized in the literature. One example of L2 self-instruction which is prevalent in Japan is learning using SIR materials, with the country's public broadcasting company offering SIR English-education materials since 1934. One problem related to L2 self-instruction is the high learner drop-out rates and the issue of motivation. Thus, in order to examine self-instructed learners' motivational bases, data collection consisted of a questionnaire and two sets of interviews which were conducted half a year apart. The data were analyzed within the framework of (a) the structure of learners' motivation within the self-determination theory (Deci & Ryan, 1985), international posture (Yashima, 2009), L2 communication confidence, and L2 willingness to communicate (MacIntyre, 2007), and (b) differences in these variables between persistent and non-persistent learners. The results indicated that intrinsic motivation and identified regulation were important variables related to persistence, as well as the amount of effort made in learning using SIR materials and the quality of learning experiences with these materials. Furthermore, simply persisting in learning using SIR materials did not make learners confident in L2 communication. Practice in L2 communication seemed necessary to be confident in L2 communication, which SIR materials did not offer. Although opportunities for L2 communication were not frequent among the participants, they found innovative ways for L2 communication, such as communicating in English with their siblings. When they were confident in L2 communication, they were likely to be willing to communicate in the L2. These results suggest that in order to help learners persist SIR materials can have sections (a) stimulating the intrinsic aspect of L2 learning and (b) showing various role models of L2 users and demonstrating the importance of L2 learning. Furthermore, learners may be able to persist by finding their own ways of engaging in L2 communication outside their studies with SIR materials, supplementing the deficiency of self-instruction. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]