As the level of technology increases, an increasing number of U.S. and U.K. universities are incorporating Computer Assisted Language Learning (CALL) into their curricula. This study investigates the potential value of an online vocabulary learning system called Memrise for memory retention and the learning of difficult, foreign vocabulary. To determine the potential value, twenty-six undergraduate English as a Foreign Language (EFL) students in a TOEFL iBT preparation course were requested to study the 400 Words of TOEFL data set on Memrise. Fifty-one most incorrectly identified words were categorized according to accuracy, and frequency. An average acquisition accuracy rate of above 75% was obtained for the majority of difficult foreign words. Although longer foreign words tended to be harder to memorize than shorter foreign words, shorter, obscure foreign words tended to be equally challenging for students. This suggest that both word length and prior language experience has an effect on memory retention. Overall, repeated exposure of difficult foreign words through spaced intervals seemed to aid memory retention and vocabulary acquisition highlighting the value of using Memrise for vocabulary learning.