Studies on how the velocity of is affected by temperature and pressure are important for understanding seismic properties of fluid and rock systems with a component. We carried out laboratory experiments to investigate velocity of in temperatures ranging from and pressures ranging from , in which is in a liquid phase. The results show that under the above conditions, in general, the velocity of increases as pressure increases and temperature decreases. Near the critical point ( and ), the velocity of reaches a minimum and has a complicated behavior with temperature and pressure conditions due to the transition between gas and liquid phases. We also developed preliminary empirical models to calculate the velocity of based on newly measured data.