Online measurements of fiber temperature and diameter were made for both the melt-spinning and the melt-blowing processes. The fiber temperature was determined by infrared thermography, and the fiber diameter was determined by high-speed photography. These measurements were then compared with predictions made with mathematical models for melt spinning and melt blowing. There was good agreement between the models and the experimental results, and the agreement was best when heat-transfer correlations developed specifically for fine fibers (cylinders) were used.