The primary aim of this investigation was to determine what combination of target word variables (frequency, patternedness, length, cognateness, lexicalization) could best predict the difficulty of incidentally acquiring vocabulary through reading. A group of adult English First Language (EL1) (n = 20) and adult English as a Foreign Language (EFL) (n = 32) learners were given a copy of an unmodified English novel containing nonce words to read within two weeks. After reading, they were given an unexpected meaning recall translation assessment measuring acquisition of 49 target nonce words. Results indicated that previously unknown frequently occurring shorter length cognates incidentally encountered are more likely to be acquired and retained by adult EFL learners and it was generally easier for EL1 learners to acquire and retain more frequently occurring lexicalized cognates. Results also indicate that the frequency of exposure to target words mattered more to the EFL and less to the EL1 learners. Correlations between patternedness and assessment results for both groups were not significant, further indicating no statistically significant effect of repeated surrounding contexts on incidental acquisition. Pedagogical and methodological implications are discussed.