This paper presents an experimental study of the influence that relative humidity and evaporation rate have on the electrospinning process in terms of fiber diameter, process measurements, and selection of operating regime (applied voltage and flow rate) for polyethylene oxide/water (aqueous) solutions and poly(vinylpyrrolidone)/alcohol (non-aqueous) solutions. Poly(vinylpyrrolidone) alcohol solutions are studied to understand the separate influence of relative humidity and evaporation rate. Correlations are developed that relate measurable process parameters (jet diameter, charge density) as well as relative humidity and evaporation rate to fiber diameter. In addition, the influence that relative humidity has on selection of operating regime to achieve desired fiber diameter and maximum production rate is presented.