This article reports on a diary study conducted to explore third language acquisition in the immersion setting in Mexico as experienced by a bilingual Russian/English learner of Spanish who had recently immigrated to the USA from Russia. Language learning is approached as a socialization process that involves negotiation of meaning (Vygotsky, 1978) and identities (Gee, 1999; Norton, 2000). The study reveals links between self-assessment, autonomy and identity, documenting how the learner utilized her bilingualism as an important resource in learning Spanish, shifted between languages and cultures and employed a variety of self-assessment techniques. The findings of the study suggest that immersion in a new language could make a powerful impact on how bilingual immigrant learners self-interpret themselves in their first and second language. The study emphasizes the contextual nature of identity and highlights the importance of introspection as an effective method of raising critical language awareness.