A study of the crystallization behavior of poly(ethylene coacrylic acids) and poly­(ethylene comethacrylic acids) and their lithium, sodium, calcium, and cesium salts has been made utilizing the techniques of X-ray diffraction, differential scanning calorimetry, and infrared spectroscopy. The acid substituent content ranged from 2.1 to 6.3 mol %. The results indicate that the acid substituents do not incorporate into the polyethylene crystal lattice and that these substituents have large effects on the kinetics of crystallization, especially in the case of the salts. The results are interpreted on the basis of a three-phase model consisting of a polyethylene crystal phase, a mixed amorphous phase, and a micro­phase-separated ionic cluster phase.