Children learning two languages (Dual Language Learners; DLLs) represent a rapidly growing population in the United States. DLLs are disproportionately more likely to live in families of low socioeconomic status (SES), which places many of them at risk for poor dual-language outcomes. To date, most studies on SES and dual language development have relied on static measures of vocabulary and syntactic skills, without examining the language learning processes -- children's ability to acquire new language items. The current study used a newly developed language measure, the Quick Interactive Language Screener: English and Spanish (QUILS:ES), to assess 3- through 5-year-old Spanish-English DLLs' language learning processes. We also examined the association between SES (as measured by primary caregivers' education) and language learning processes, and further explored mechanisms underlying the association. DLLs from higher-SES families showed better language learning skills than those from lower-SES families. The size of the gap did not vary by child age. Home literacy environment (i.e., access to books, book-reading frequency) and children's existing knowledge (i.e., vocabulary and syntactic knowledge) mediated the SES effect. Together, these findings highlight the need to better prepare DLLs from low-SES families for learning from a dual-language environment. Supporting DLLs' language environment and knowledge through learning materials and language and literacy activities in both languages is crucial for ameliorating the SES gap in language learning processes.