We investigate the determinants of US swap spreads based on the development of the swap market and the major events that happened between 1991 and 2006. We find that changes in swap spreads are jointly determined by the liquidity premium, interest rate level, default risk premium and the business cycle. The changes in swap spreads are positively related to liquidity premium, interest rate level and the slope of risk-free term structure. Amongst the various credit spreads, Finance AA spreads and agency spreads have the most influence on swap spreads. We also find that swap spreads changed from pro-cyclical to counter-cyclical after 1999. When the market features heavy speculative trading, such as the convergence trading activities of swap sp...