This paper investigates determinants of transfer enrollment within four-year institutions. After outlining probable determinants (e.g., enrollment supply and demand and financial considerations), data from a national sample of institutions from 1984-1997 are used to examine whether these factors explain the composition of a school's enrollment. Next, the paper examines variation in enrollments across institutional types and finds three main differences: (1) the transfer enrollment rate, the percentage of an institution's incoming students who are transfers, is larger at public than private institutions; (2) the transfer enrollment rate falls for private institutions as they become more selective, but the relationship between selectivity and transfer enrollment share is more complex for public institutions; and (3) private liberal arts I colleges have lower transfer enrollment rates than other schools. Finally the paper investigates factors besides institutional type that determine the composition of a school's new student enrollment. For both public and private institutions, transfer enrollment rates are higher at institutions with more student attrition, less financial resources, less freshmen in campus housing, lower tuition and fees, and more students attending two-year institutions in the school's state. (Contains 17 references.) (SM)