This study tested whether interpersonal complementarity would mediate the relationship between birth rank complementarity (i.e., spouses have different birth ranks in their family of origin) and marital adjustment, which was originally proposed by Toman's (1993) duplication theorem of social relationships . Birth rank complementarity was indexed by categorizing couples based on the birth rank of each spouse. Interpersonal complementarity was operationalized by creating composite scores from spouse ratings of him/herself on the SASB Intrex scales, which measure interpersonal behavior in both the active and response roles of an interaction. Marital adjustment was operationalized by creating composite scores from spouse ratings on two subscales of the Marital Satisfaction Inventory-Revised (Snyder, 1997): Global Distress, which measures general satisfaction with the marital relationship; and Problem-Solving Communication, which measures a couple's ability to effectively resolve conflict. The sample was comprised of 60 heterosexual couples who were married between January and December of 2001, 2002, or 2003. The main hypothesis for the study, that interpersonal complementarity would mediate the relationship between birth rank complementarity and marital adjustment, was not supported: the direct relationship between birth rank complementarity and marital adjustment was not significant. However, findings indicated that spouse composite ratings of interpersonal complementarity predicted spouse marital adjustment, and birth rank complementarity was found to be associated with interpersonal complementarity. Directions for future research, study limitations, and implications for practice are discussed.