The implications of the entanglement concentration (C-e) on the electrospinning process for a series of linear and branched poly(ethylene terephthalate-co-ethylene isophthalate) (PET-co-PEI) copolymers with weight-average molecular weights (M-w) ranging from 11700 to 106 000 g/mol and branching index values (g') from 1.0 to 0.43 were investigated. Analyzing the dependence of specific viscosity (eta(sp)) on concentration enabled the determination of the semidilute unentangled, semidilute entangled, and concentrated regimes for the PET-co-PEI solutions. Linear and branched copolymers were electrospun from semidilute unentangled, semidilute entangled, and concentrated solutions under identical conditions to determine the effects of concentration regime and molecular topology on electrospun fiber morphology. The dependence of the fiber diameter and morphology on the zero shear rate viscosity (etao) and normalized concentration (C/C-e) was determined. For copolyesters with molecular weights well above the entanglement molecular weight, C-e was the minimum concentration required for electrospinning of beaded fibers, while 2-2.5 times C-e was the minimum concentration required for electrospinning of uniform, bead-free fibers. When the concentration was normalized with C-e, the influence of chain length and topology on the electrospinning process was removed, and the fiber diameter universally scaled with the normalized concentration to the 2.6 power.