This paper examines the effects of the post-merger integration duration on acquiring firms’ leverage behavior before and after a merger, using a dynamic model in which full merger benefits cannot be consumed at the instant of a merger, but rather after a prespecified post-merger integration period. The model generates new implications related to acquiring firms’ leverage dynamics along with method of payment choice. Specifically, the model indicates that the post-merger integration duration is negatively associated with the market leverage of newly-merged firms at the time of merger completion and during the integration period. Further, acquirer managers are more likely to use equity to finance a merger when the integration duration is likely to be lengthy. Our empirical tests provide evidence consistent with the model implications.