Purpose: This study investigated how word characteristics (frequency, cognate status, acquisition source, translation equivalence) influence vocabulary development in Spanish-English bilingual children. We examined (a) how second language (L2) exposure relates to lexical development in first language and L2 over time and (b) which lexical characteristics influence lexical development across languages. Method: We examined item-level vocabulary responses of Spanish-English bilingual children (n = 225, ages 5-12 years) tested on the same set of expressive vocabulary items in both languages across four annual time points. Using binomial generalized mixed-effects models, we analyzed how acquisition varied by word characteristic (frequency, cognate status, acquisition context, translation equivalence) while controlling for age and language exposure. Results: Key findings included that (a) high-frequency words showed greater odds of acquisition in both languages, (b) cognates demonstrated facilitative effects primarily in English, (c) home-acquired words were more readily learned than school-acquired words in both languages, and (d) knowledge of translations boosted acquisition in the other language. Conclusions: The findings support usage-based theories of language acquisition and demonstrate the integrated nature of bilingual lexical development. These results have important implications for bilingual education, suggesting that supporting first language development facilitates rather than hinders second language acquisition.