We have extensively studied the nonequilibrium behavior of atactic polystyrenes in a broad range of temperatures. Depending on the conditions of the initial temperature (To) and the aging temperature (T), the shapes of isotherms plotted in the form of total fractional free volume (viz., δ) versus logarithmic aging time can be linear or nonlinear. At aTin the vicinity ofTg, the isotherms converge together. When theTislowered, the isotherms become increasingly nonlinear; eventually, these isotherms intersect one another at various points, forming a newly discovered crossing phenomenon. Differences in volume contraction persist throughout further aging, regardless of the equality in δ right at the moment of crossing. In a further case, the nonlinear isotherms between the two polystyrenes may be superposable by a simple δ shift, despite their substantial differences in δ. As none of these phenomena can be justified by kinetic arguments, this study seriously questions the validity of the existing kinetic theories as well as the existence of the extrapolated underlying equilibrium state.