Electrospun polymeric fibers are attractive candidates in the development of scaffolds for the tissue engineering and for providing new systems for delivery of bioactive molecules. Co-axial fibers have emerged as an efficient tool to protect the core material from the adverse conditions of electrospinning process, to spin difficult-to-process fluids and to generate fibers with much more control of the delivery of encapsulated bioactive molecules. Currently, there is very little reported work on the optimization of the processing parameters of electrospinning, especially core-shell electrospinning. This study extends the understanding of the role of solution viscosity as a vital material parameter for electrospinning of fibers. The spinning solutions were characterized for viscosity and optical imaging of the compound Taylor cone for spinnability, and the fibers were imaged by Scanning Electron Microscopy (SEM). Our experimental results, using PLGA as the model polymer, confirm that the solution concentration be above the entanglement concentration (C-e) to obtain uniform beadless monolithic fibers; for core-shell fibers, the shell solution must fulfill the above criterion for spinnability and, further, the ratio of the viscosities of core and shell solutions (eta(core)/eta(shell)) has to be greater than a threshold value to get a stable compound Taylor cone and therefore to obtain uniform beadless core-shell fibers. Addition of surfactant led to reduction of the threshold eta(core)/eta(shell) (from 0.55 to 0.18) for the PVA-PLGA system. (C) 2012 Elsevier B.V. All rights reserved.