This article examines the effects of synonymy (i.e., learning words with and without high-frequency synonyms that were known to the learners) on word knowledge in a study of 84 Japanese students learning English. It employed 10 tests measuring 5 aspects of word knowledge (orthography, paradigmatic association, syntagmatic association, meaning and form, and grammatical functions) to assess learning. Both receptive and productive tests were used to measure each aspect of vocabulary knowledge. The participants encountered target words in 2 learning conditions: glossed sentences and word pairs. The results showed that the learners had significantly higher scores for the words that had known synonyms on productive knowledge as measured using syntagmatic association and paradigmatic association tests and on receptive knowledge as measured using an orthography test. The findings indicate that learning synonyms for known words may be easier than learning words that do not have known synonyms. A list of Glossed Sentences is appended. (Contains 2 tables.)