Electrospinning is a method to produce submicron polymer fibers for a wide range of applications. In many applications, the average electrospun fiber size and uniformity are important for the product's performance and process economics. Thus, it is desirable for electrospinning to achieve consistent and controllable fiber diameters. However, the current state-of-the-art electrospinning process can result in variable fiber diameters, both run-to-run and during a run. This paper investigates how the operating regime as well as several important process factors affect fiber diameter using a vision-based system. For aqueous polyethylene oxide (PEO) solutions, it is found that the relative humidity has a strong effect on fiber diameter. Correlations between measurable parameters and fiber diameter are also developed to provide the ability to achieve the desired fiber diameters. The jet dynamics are experimentally identified through step response for development of appropriate control strategies.