This study measured the effects of an online supplementary mathematics curriculum designed for middle school English language learners who speak Spanish as a first language. A randomized experiment measured the achievement differences between middle school English language learners who used the Web-based HELP Math (Help with English Language Proficiency) curriculum and students who used other technology-based programs. Three hundred and ninety-six students participated. Both groups made statistically significant gains from pretest to posttest within their respective curricula, but no main effect was found between the two groups. Post hoc analyses revealed that students with higher levels of English proficiency, who participated in the comparison condition, performed significantly better than students in the HELP Math condition, while students with lower levels of English proficiency performed better in the HELP Math program (although these differences were not statistically significant). Findings are interpreted with caution due to the truncated length of the intervention.