A mixture of low-molecular-weight poly(L-lactide-co-glycolide ethylene oxide fumarate) (PLGEOF) macromer and high-molecular-weight poly(lactide-co-glycolide) (PLGA) was used to produce fibers by electrospinning. PLGEOF is a biodegradable and in situ cross-linkable terpolymer made from building blocks with excellent biocompatibility. PLGA provides the required elongational viscosity to the spinning jet while the unsaturated PLGEOF macromers contribute to in situ cross-linking of fibers and attachment of bioactive functional groups. Mechanical rheometry demonstrated that PLGEOF macromers cross-link in situ by ultraviolet radiation. The addition of PLGEOF macromer to PLGA solutions had a significant effect on size and morphology of the electrospun fibers. The morphology of the electrospun fibers changed from bead- to fiber-like with increasing PLGEOF concentration. As PLGEOF was added to 12 wt% PLGA solutions, the fiber diameter first decreased with 2% PLGEOF and then increased with the addition of 5% and 10% PLGEOF. Our results demonstrate that the fiber size initially is decreased with the addition of PLGEOF due to an increase in solution conductivity and then is increased with further PLGEOF addition due to higher viscosity of the polymerizing mixture.