This study examined the Spanish and English spelling patterns of bilingual adolescents, including the cross-linguistic effects of each language, by applying a fine-grained measure to the differences in spelling in naturalistic writing. Spelling errors were taken from narrative and expository writing samples provided by 20 Spanish-English bilingual adolescents (n = 160). Errors were coded by categories (phonological, orthographic, and morphological) and specific linguistic features affected and then analyzed by language and genre. Descriptive analyses noted similarities and differences among error patterns in both languages as well as language transfer (i.e., borrowings and code-switching). Statistical analyses revealed language differences in proportions of misspellings across linguistic categories. More fine-grained analyses indicated linguistic feature patterns that were shared across languages and unique to each language. Finally, borrowing, while infrequent, was noted more frequently in English compositions. This investigation appears to demonstrate that spelling, when approached as both a cognitive and linguistic activity, is complex since multiple knowledge systems must be coordinated. The use of triple word form theory to analyze misspellings in emerging bilingual writers suggests that discerning patterns of misspellings in each language provides more insight than does transfer alone into the extent that phonology, orthography, and morphology are becoming unified.