There are many methods to accelerate the aging process of asphalts to determine how susceptible the asphalt is to hardening. These tests use increased temperature, pressure, or both, and assume that the properties after the accelerated test match those if the asphalt is aged at lower temperature and pressure. However, the slope of the logarithm of viscosity versus carbonyl area, known as the hardening susceptibility, does not easily correlate from high-pressure to low-pressure aging conditions. The hardening susceptibility (HS) is a strong function of the oxygen pressure at which the test is run. HS is a function of pressure because the two terms that compose itthe asphaltene formation susceptibility, which determines how susceptible the asphalt is to the production of asphaltenes, and [Formula: see text], which determines how the asphaltenes are affecting the viscosity of the asphaltare functions of pressure. The pressure dependency is hypothesized to be caused by oxygen diffusion on a molecular scale.