This study aims to understand the learning experience of English language learners (ELLs) within the framework of Kuhlthau's Information Search Process (ISP). Forty‐eight ELL students from three classes at a high school participated in the study while they conducted a research project in English. Data were collected through demographic questionnaire and process surveys. Students' demographic information, knowledge about their research topic, labeling of knowledge, estimate of interest and knowledge, and learning outcomes were collected and analyzed with content analysis and statistical techniques. The findings indicate that ELL students, as a whole group, showed significant increases in their topical knowledge and estimate of interest and knowledge as they progressed in the research project, which are consistent with what other ISP‐based studies found. When three different English proficiency‐level groups were compared, only the intermediate group showed significant increases in topical knowledge and estimate of knowledge throughout the process. Also, different research tasks impacted the amount and substance of knowledge students built and their estimated knowledge during the research project. The findings led to suggestions for instructional strategies such as learning goals reflecting various kinds of learning, differentiated instructions in mixed‐ability classrooms, structured interventions, personalized research topics, and teacher–school librarian collaborations.