A universal temperature-pressure superposition principle found for compression data of liquids is discussed in terms of a recently suggested isothermal equation of state. The equivalence of both treatments is outlined. New relationships relating a pseudospinodal curve with the zero-pressure value (B-0) and the pressure coefficient (B-1) bf the isothermal bulk modulus (B) are introduced. Values of B-1 directly obtained from our treatment agree well with experimental values found for some organic liquids and water in the normal liquid range and corroborate that the isotherms of kappa(T) = B-1 can be described for most liquids by an inverse power law of the pressure with exponent close to 0.85.