The research on incidental vocabulary acquisition through reading has claimed that repeated encounters of unfamiliar words and the context in which these words occur facilitate word learning. However, so far both variables have been investigated in isolation. This study was conducted to explore the impact of context types, word occurrence frequencies and the combination of these variables on learning and retention of unknown words. The context types included both more and less informative contexts. The word occurrence rates were 1, 5 and 10 times. A total of 180 Chinese university students participated in the study, with 30 in each of the 6 conditions. Learners in each condition were exposed to the same 15 target words. The more informative context was found to have significant effects on productive and receptive knowledge of meaning while an increase in word occurrences was found to produce significant effects on productive and receptive knowledge of word form. With regard to acquisition of word meaning, the cumulative effect of repeated encounters with new words was found only in the more informative context. Providing learners with 10 encounters of new words in an informative context is realistic for triggering the incidental vocabulary acquisition.