New words were embedded in high- and low-constraint sentences and presented three times in a random order to adult learners of Chinese as a second language. The learners explained the meaning of each word in their native languages and their answers were scored by other native speakers. The overall accuracy was .47 with no effect of constraint or frequency. When the data were limited to those words the learners reported having no prior encounters with and those sentences they reported comprehending, the accuracy was .45. The results demonstrated fast mapping of word learning in adult L2 learners but indicated that extended mapping was necessary to achieve ultimate attainment. The results are also consistent with Krashen’s (1982) “comprehensible input” and “i+1” hypothesis.