The purpose of this study was to examine the relationship between measures of written vocabulary and writing quality. Participants included 92 second-grade students and 101 fourth-grade students. Students completed two writing samples: one an experimenter-developed writing task and the other, a standardized assessment of writing quality. Research questions examined whether four vocabulary measures (vocabulary diversity, less frequent vocabulary, mean syllable length, number of polysyllabic words) demonstrated developmental differences, whether the vocabulary measures remained stable across two different writing prompts, and whether the vocabulary measures explained unique and shared variance beyond that explained by compositional length and compositional spelling. The results indicated that vocabulary diversity and less frequent vocabulary showed developmental differences across the two writing tasks. Vocabulary diversity was the only variable to remain stable across the two writing tasks. Commonality analysis revealed that vocabulary measures explained unique and shared variance in writing quality in all four models (2 grades and 2 writing prompts). Generally, vocabulary diversity was the most stable and consistent of the four vocabulary variables.