A daily return reversal measure of liquidity is developed and estimated using a new comprehensive ultra-high frequency data set of foreign exchange rates during the financial crisis period of 2007--2008. The measure captures market participants' perception of periods with high and low liquidity in the expected manner. Tests for commonality in foreign exchange (FX) liquidity show that liquidity co-moves strongly across currencies. Systematic FX liquidity decreases dramatically during the subprime crisis, especially after the default of Lehman Brothers in September 2008. To investigate whether there exists a return premium for illiquidity, a factor model similar to Lustig, Roussanov, and Verdelhan (2009) is augmented by a liquidity risk factor constructed from shocks to systematic liquidity. Empirical results indicate that liquidity risk is a heavily priced state variable important for the determination of FX returns. Previously identified risk factors such as the carry trade and market risk factors are no longer significant once common liquidity risk is incorporated in the asset pricing model. This finding helps to explain deviations from uncovered interest rate parity as classical tests do not include liquidity risk.