Extramural English (EE) exposure has been shown to correlate with general vocabulary knowledge. It remains uncertain, however, how academic vocabulary knowledge correlates with EE and can be explained by EE factors and demographic factors. Therefore, an academic vocabulary test, a background questionnaire, and a survey on current EE involvement were administered to 817 Swedish upper-secondary students in university-preparatory study programmes. A linear model revealed little explanation from demographic factors (age, gender, number of first languages, length of English instruction, and parental educational level) whereas EE factors (reading, listening & viewing without textual support, viewing with Swedish subtitles) accounted for 26% of the variation. Since extensive EE involvement may support the incidental learning of academic lexis, the paper suggests pre-tertiary instructional principles being guided by extramural as well as intramural incidental learning opportunities.