<p>Defects in materials form, interact, and evolve at the atomic level. The measurements and analysis of their atomic level generation and evolution can reveal the mechanisms of defect interactions which dictate the macroscopic material properties and strength. Here we investigate atomic scale defects in multi-principal element alloys (MPEAs) of four and five elements. We combine positron annihilation spectroscopy (PAS) with a rigorous analysis of the phases, microstructures, and chemical compositions by x-ray diffraction (XRD), scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDX). PAS reveals an interesting phenomenon in which ion irradiation induced defects substantially reduce the size of the original vacancy clusters in alloys of specific chemical compositions. The study shows the strong impact of the nature of chemical disorder on atomic scale defect interactions and evolution during growth and irradiation. Replacing one element or changing its percentage modifies the phase and segregation and strongly impact defect formation. Depletion of Ni from the matrix enhances defect formation and clustering and hinders the alloy radiation tolerance while the formation of dendric structure reduces point defect formation and improves radiation resistance. This study manifests the complexity of the interaction between the chemical arrangement of atoms and defect migration and evolution in MPEAs and demonstrates that defect interactions are driven by phase nature, phase separation and segregation of elements.</p>