Aims and objectives: L2 learners who speak a null subject (L1 Spanish) and a non-null subject language (L2 English) may experience cross-linguistic interference from the L2 on L1 pronoun interpretation. In this study, we test pronoun interpretation in the L1 and L2 of adult learners, in comparison with two groups of monolingual speakers, to assess if L1 pronoun interpretation can change as a result of L2 acquisition at intermediate-advanced levels of L2 proficiency and in the absence of long L2 immersion. Methodology: A group of L2 English speakers (L1 Spanish) participated in two offline sentence comprehension tasks where they interpreted pronouns in the L1 and L2. The results are compared with English and Spanish monolingual speakers. Data and analysis: We find that adult English learners comprehend pronouns in their L1 (Spanish) differently than Spanish monolingual speakers, demonstrating a strong subject bias for interpreting null and overt pronouns. In addition, we show that pronoun interpretation patterns acquired in the L2 explain the changes to L1 interpretation biases. Conclusions: The results of this study significantly advance the understanding of the factors that contribute to bilingual language comprehension, showing the permeability of the L1 comprehension system at the discourse level as soon as the L2 sets in.