This study explores the lexical profile of essays written by 48 advanced learners of second language (L2) Russian who participated in the Russian Overseas Flagship, an intensive year-long study abroad program designed to help students reach Interagency Language Roundtable (ILR) Level 3 proficiency in all skills. Using the lexical frequency profile and P-Lex as measures of vocabulary sophistication, the study found that over the 9 months of the program, students significantly increased their usage of words from the lowest frequency bands. This adds to previous findings that knowledge of lexical items at the 3,000-5,000 word frequency levels predicts reading proficiency at the ACTFL advanced high-superior level in Russian. However, the increase of vocabulary sophistication was not clearly correlated with improvements in the students' writing proficiency scores, as measured on the ILR scale. A qualitative analysis of the students' low-frequency vocabulary usage reveals their control of native Russian vocabulary and derivational morphology. The analyses reveal the effects of writing tasks on student vocabulary usage.