It is commonly believed that passionate love diminishes over time even as companionate love may grow. According to the self-expansion model (Aron and Aron (1986) Love and the expansion of the self: Understanding attraction and satisfaction. New York, NY: Hemisphere Publishing Co/Harper & Row Publishers), this change may reflect changes in opportunities for self-expansion in the relationship. Early in relationships, as partners continuously learn new things about each other, self-expansion—which occurs through the integration of the qualities and characteristics of the partner into oneself—generates passion for one’s partner. Later, as one’s partner is completely included in the self, self-expansion opportunities diminish and less passion is generated. From this perspective, if self-expansion could be sustained, so could passionate love. Over 500 adults in a Midwestern community responded to a telephone survey about their romantic relationship. They answered questions about the length of their relationship, experiences of self-expansion within their relationship, passionate love—including both romantic and obsessive components—and companionate love. As expected, people in longer lasting relationships reported lower levels of self-expansion within their relationships. Romantic and obsessive components of passionate love showed different patterns across time in relationship, but both were positively associated with self-expansion experiences as expected by the model. Self-expansion was unrelated to companionate love. Broadly consistent with the self-expansion model, these findings highlight a need for further theoretical development to explain the specific linkages of self-expansion with different components of passion.