For advanced engine and heat-exchanger design, it is essential to know the critical points of the fuel and fuel-product mixtures. Ultrasonic velocity measurements provide critical-point information, and can also be used to obtain compressibilities. A novel system has been designed and constructed to measure the velocities of sound at temperatures up to 400 degrees C and pressures to 300 bar. A quartz crystal has been used to generate and receive the ultrasonic waves. Methylcyclohexane is used as a model for new endothermic fuels, and critical points have been measured for methylcyclohexane and mixtures of methylcyclohexane and benzene. Experimental velocities and critical points are compared to values calculated from a Benedict-Webb-Rubin equation of state, the corresponding-states principle, and mixing rules.