Aims and Objectives: The current study’s aim was to test the ambiguity and dominance theories of transfer by examining compound noun production and comprehension by bilinguals acquiring Spanish and Japanese, as the word order of nominal compounds in these languages is always reversed, making them grammatically and theoretically unambiguous. Methodology: Ten Spanish-Japanese bilingual preschoolers completed production and comprehension elicitation tasks. Data and Analysis: The research subjects’ reversal rates were compared with those of age- and vocabulary-matched Japanese monolinguals. Findings/Conclusions: The results demonstrate that transfers occur from Spanish to Japanese in both production and comprehension, and that there are no dominance effects on the degree of cross-linguistic influence. Originality: There have been no previous studies on cross-linguistic transfer in Spanish-Japanese bilingual children. Significance/Implications: Transfer and directionality are not affected by relative vocabulary level; the concept of dominance should be (re)considered carefully especially for young bilinguals whose language inputs are greatly imbalanced and variable. Moreover, what is considered grammatically unambiguous by adults may be ambiguous for children acquiring such knowledge bilingually, which raises the need to consider structures in both languages as affecting the acquisition of language in young bilinguals.