This study investigated the relationship of first language (L1) grammatical knowledge to English second language reading (ESLR), with the objective of understanding this relationship in the context of the transfer of L1 skills to second language (L2) academic processes. Fifty-five adult, native Spanish-speaking English-language learners were given 4 assessments. Spanish reading accounted for 10% of the variance of ESLR, supporting the Linguistic Interdependence Hypothesis, and English grammar accounted for 8% of the variance, supporting the Linguistic Threshold Hypothesis. The results imply that transfer from the L1 might operate differently in adult ESLR than it does with children. An ESLR adult model is proposed, which predicts that some ESLR students will require a curriculum that provides a highly intensive focus on L2 language, grammar, and reading skills. (Contains 2 tables and 2 figures.)