Byline: Judith G. Kelley, Jon C.W. Pevehouse The United States often leads in the creation of treaties, but it sometimes never joins those treaties or does so only after considerable delay. This presents an interesting puzzle. Most international relations theory expects states to join treaties as long as the benefits outweigh the costs. Domestic theories modify this with the constraints of institutional veto players. Yet, sometimes neither of these arguments explains the delay or absence of US participation. We supplement these explanations with an opportunity cost theory. We argue that the advice and consent process sometimes slows or stalls because it imposes costs in terms of legislative time and political capital. These costs alter the calculus of key players and may obstruct the process. Statistical analysis supports the argument. The priority the Senate and President give to treaties depends not only on the value they assign to the treaty, but also on the value of the time needed to process the treaty. Presidents are less, not more, likely to transmit treaties to the Senate the more support they have in Congress. Furthermore, the more support the President has in Congress, the more the cost of Senate floor time matters for advice and consent. CAPTION(S): Table S1. Descriptive statistics. Table S2. Robustness checks of the transmittal model. Table S3. Robustness checks for advice and consent model. Table S4. Senate-only robustness checks for interactive models. Figure S1. Hazard rate for treaties signed by the US transmitted to the Senate. Figure S2. Hazard rate for advice and consent of treaties transmitted to Senate. Figure S3. Marginal effect of President Percent Control, conditional on Implementation. Figure S4. Marginal Effect of President Percent Control, conditional on Election.