Asphalt concrete mixtures with different levels of oxidative aging, prepared by oven-aging the mixture at 135 oC for different amounts of time, were used to study the effects of oxidative aging upon the ultrasonic phase velocities and attenuation measurements. It was a observed that both the dilatational and shear velocities increase up to approximately 24 hours of aging after which they significantly decease with aging. Also, both the dilatational and shear attenuation decrease up to around 24 hours of aging, after which both attenuations strongly increase. These results are consistent with results obtained using the mechanical Disk-shaped Compact Tension (DC[T]) fracture tests. Based upon these velocity and attenuation measurements, the dynamic moduli were calculated. It was observed that the dynamic moduli increase from 0 hours to 24 hours and decrease from 24 to 36 hours of oven-aging. The modulus obtained using ultrasonic measurements is also compared with the modulus obtained using the AASHTO recommended mechanical testing. The differences are due to scattering effects, which are present in ultrasonic testing. It was also observed that to avoid the uncertainty associated with assuming a suitable value for the Poisson's ratio, both the dilatational and shear velocities and corresponding attenuation measurements must be carried out. Furthermore, to eliminate the need for traditional mechanical testing during estimation of complex moduli, frequency-dependent ultrasonic measurements must also be carried out.