The study reported in this article describes second language (L2) vocabulary learning outcomes associated with adult L2 reading comprehension processes, thus connecting L2 learning with the complex cognitive and linguistic processing involved in reading. The study aimed to determine whether background knowledge moderated the relationship between passage comprehension and lexical input processing outcomes, such as intake and receptive gain and retention of target-word meanings. The primary theoretical contribution concerns the nature of the relationships that obtained between passage comprehension and lexical input processing. Overall, the results suggest that as learners become more efficient in engaging in the various processing activities required during L2 reading, they experience greater memory for linguistic elements encountered during reading, such as orthographic forms and semantic aspects of new lexical items. The results support connectionist models of L2 reading and conclusions concerning efficiency in lower level text processing (e.g., see Koda, 2005; Nassaji, 2002). Reading is a complex cognitive activity, involving simultaneous linguistic processing such as pattern recognition, letter identification, lexical access, concept activation, syntactic analysis, propositional encoding, sentence comprehension, and intersentence integration, as well as the activation of prior knowledge, information storage, and comprehension monitoring. The prior knowledge that is accessed is largely determined by the quality of the textbase that is constructed during reading, which is affected by the individual's efficiency in carrying out the various text-processing operations listed above. The finding that background knowledge did not moderate the relationship between comprehension and receptive retention of meaning provides further support for the robust role of text-processing efficiency in bootstrapping processes, such as those involved in mapping new linguistic forms to familiar concepts already stored in memory. The secondary finding with regard to intake of the target words corroborates the main finding, demonstrating that efficient text processing skills are also essential to other aspects of lexical input processing, such as establishing and retrieving the connections between new linguistic forms and the specific contexts in which they were encountered. The study also expands upon previous reading and research in the area of methodological innovations applied to incidental learning research paradigms, for instance, by measuring intake and episodic memory and the relationships between text processing and language learning outcomes. In addition, through the concurrent investigation of the impact of several factors on lexical input processing, the study sets the stage for more complex modeling of the processes that contribute to L2 development.