Surface tension data can be used for estimating the solubility of polymers in liquids. By determining the apolar and the polar components of the surface tension of polymers and of solvents, the attractive free energy, δG121, of a polymer (1) in a given solvent (2) can be established. By also taking into account the contactable surface area of two polymer molecules, immersed in a liquid, δG121can be expressed in units of kT. Solubility then is favored when -1.5 kT < δG121< 0 for apolar systems, and when -1.5 kT < δG121for polar systems. In polar solvents, hydrogen bonding can often increase δG121from <-1.5 kT to > + 1.5 kT. Positive values are frequently attained and this strongly shifts the behavior from insolubility to solubility. A number of proteins exemplify this behavior.