The expansion of private sectors of higher education has usually been regarded as a factor of diversification in higher education systems. Some of this differentiation has been found to arise from the affiliation of private institutions with organizations outside the field of higher education. This article reports the results of a study of this form of interorganizational relationship in private universities in Chile. Cases include universities founded or sponsored by religious, business and military organizations. A typology of private universities is proposed, on the basis of the forms affiliation (or its absence), was observed to take in the cases examined. Weak and strong forms of affiliation are described, and affiliated universities are compared to "proprietary" universities, i.e., those owned by individuals who govern them from their positions in the board of directors, and "independent" universities, in which governance lies with internal--academic or administrative--constituents. Albeit derived from the case of Chile, the typology could be applied to the analysis of private higher education in other national systems. The second part of the article seeks to ascertain whether affiliation operates as a source of differentiation in Chilean private higher education. Results show that, compared to the other types of private universities, the affiliated ones possess distinctive mission statements and declarations of principles, consistent with the orientations of their sponsor institutions, they tend to be smaller, and to have more full-time and better qualified faculty. Some receive financial support from their sponsor organization or its members. Distinctiveness was not found in indicators of prestige and student selectivity, nor in tuition levels, program offerings, curriculum design, the weight of research and graduate programs in their functions, student socioeconomic profile, and faculty involvement in governance. This is not to say that there are no differences in these dimensions among private universities: much diversity exists, but most of it cuts across all categories of interest for our study. Overall, affiliation does not appear to be a strong factor behind the diversification that exists in the Chilean private university sector.