This study investigated the effectiveness of group work (GW) in EFL vocabulary learning by second year, non-English major, university students in Taiwan, in comparison with working individually (IW). The students (N = 44) worked in mixed ability groups of 3-4 or in IW to complete vocabulary exercises following reading activities. The classroom intervention followed a repeated measures design with alternating sessions (one week IW, one week GW) for 12 weeks. In order to measure students' word knowledge gains, the modified vocabulary knowledge scale was used in pre-, post- and delayed-post tests, and the scores from the tests were analyzed with paired "t" tests. Qualitative information about vocabulary discovery and retention was further obtained from interviews with 24 students conducted after the classroom intervention. Results showed that students' overall improvement in vocabulary knowledge with group work was significantly higher than that with individual work on immediate post-tests, though both treatments had a beneficial effect. Later retention of word knowledge after GW was only 2% higher than that with IW. Interpretations and implications of these findings are discussed.