This study investigates the comparative effects of presenting the new vocabulary items to students in semantically related groups vs. semantically unrelated groups on vocabulary receptive acquisition in long term memory. Two groups of participants studying at the Iran Language Institute (ILI), a well-known second language school, in Isfahan, Iran, were presented twenty new vocabularies in short reading texts, one group in semantically related sets and the other group in semantically unrelated sets. The results of the study indicate that the group which was taught the semantically unrelated sets of words outperformed the group which received semantically related sets of words in its immediate and delayed post-tests. The findings of this study provide evidence that presenting new words to students in semantically related sets may hinder long-term receptive vocabulary acquisition due to interference of similar words with each other. Learners have to discriminate between the properties of the similar words and this can impede their learning and retention.