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Parallel cohorts of Hebrew speakers learning English in the U.S., and American-English speakers learning Hebrew in Israel were tracked over the course of two years of immersion in their L2. We utilised a functional MRI semantic judgement task with print and speech tokens, as well as a battery of linguistic and cognitive behavioural measures prior to and after immersion, to track changes in both L1 and L2 processing. fMRI activation for print tokens produced a similar network of activation in both English and Hebrew, irrespective of L1 or L2 status. Significant convergence of print and speech processing was also observed in both languages across a network of left-hemisphere regions joint for both L1 and L2. Despite significant increases in behavioural measures of L2 proficiency, only a few signs of longitudinal change in L2 brain activation were found. In contrast, L1 showed widespread differences in processing across time, suggesting that the neurobiological footprint of reading is dynamic and plastic even in adults, with L2 immersion impacting L1 processing. Print/speech convergence showed little longitudinal change, suggesting that it is a stable marker of the differences in L1 and L2 processing across L2 proficiency. • Cohorts of L2 (Hebrew/English) learners were tracked over two years of immersion. • fMRI scans probed neurological changes in L1 and L2 print and speech processing. • Greater proficiency in L2 led to few longitudinal changes in L2 brain activation. • L1 processing remains plastic in adults and is impacted by L2 immersion. • Print/speech conversion is a longitudinally stable marker of both L1 and L2 reading. [ABSTRACT FROM AUTHOR]