This study examines the nature of the involvement load (Laufer & Hulstijn, 2001) in second language (L2) lexical input processing through reading by considering the effects of the reader-based factors of L2 reading proficiency and background knowledge. The lexical input processing aspects investigated were lexical inferencing (search), attentional allocation during target word (TW) verification (evaluation), TW episodic memory after verification, TW retention, and changes in lexical input processing. This study also addresses the relationships between inferencing and retention as well as the relationships among the various phases of lexical input processing. A questionnaire on self-reported strategy use during the TW verification task was designed to examine motivation and cognitive involvement. A repeated-measures design was used with a cross-sectional sample of 35 adult L2 learners of Spanish. Participants read narratives (one more and one less familiar) that contained nonsense words. They guessed the meanings of the TWs. After reading all of the stories, participants completed an online inference verification task whose goal was to confirm or to correct guesses and to encourage deeper processing of TWs. This was followed by a test of TW episodic memory (yes/no verification) and a strategy questionnaire. Afterward, participants completed two tests that measured receptive retention of meaning. Analyses reveal differential effects of the reader-based variables across the different aspects of lexical input processing and negative changes in lexical input processing due to an interaction between reading proficiency and background knowledge. Results illustrate how reader-based factors also affect learner involvement and lexical input processing.