In an extrusion process, linear viscoelastic properties of molten poly( ethylene vinyl acetate) (EVA) copolymers, which have one principal factor of variation, can be estimated from in-line near-infrared (NIR) spectra. The NIR transmission spectra of molten polymer flow stream were collected in a flow cell attached to a single-screw extruder. Dynamic rheological functions obtained from linear viscoelastic measurements, for example, the complex viscosity response, are regressed against the NIR spectra. The primary method for the rheological measurements involved sinusoidal, oscillatory shear experiments at varying angular frequencies using a cone-and-plate viscometer. All measurements were carried out on molten EVA polymers at 200 degrees C. Calibration models were built on spectra in the carbon--hydrogen (C--H) vibrational stretch, first overtone, wavelength region (1620-1840 nm), and these models were used to predict the rheological material functions of copolymer samples. The robustness of these models was tested on independent prediction samples that had not been included in the calibration models.