Like all learners, undergraduate research interns bring to their research internships a variety of initial ideas, opinions, expectations, beliefs and attitudes about research internships. However, there is little published research on students' preconceptions about research internships and the relationships of these preconceptions to actual experiences. There is also a dearth of information about potential influences of preconceptions on the learning process and gains that accrue to students involved in undergraduate research experiences (UREs). Using qualitative data from twenty five undergraduate research interns' reflective journals, the current study examines students' preconceptions of UREs and how those preconceptions compare with students' actual experiences. The analysis showed that prior to their research experiences, participants imagined research environments/laboratories as stern places devoid of social interactions. Interns also held preconceived traditional stereotypical views of their faculty mentors as scientists. Other preconceptions included the expectation of extensive one-on-one mentoring from faculty mentors and the preconception of research work as team rather than independent work. Further, the study found that students' preconceptions were mostly contradicted by their experiences in their research internships. The limitations of the study and the practical implications of the findings for devising meaningful and effective undergraduate research experiences are discussed. (Contains 1 table.)