An on-line sensor using near infrared (NIR) spectroscopy is developed for monitoring CO2 concentration in polymeric extrusion foaming processes. NIR absorption spectra are acquired by a probe installed at the foaming extruder die. The calibration curve relating the absorbance spectrum at 2019 nm to the dissolved gas concentration is derived so as to infer dissolved CO2 gas concentration on-line from measured NIR spectra. Experimental results show the developed on-line NIR sensor can successfully estimate dissolved CO2 concentration in the molten polymer and illustrate that the developed MR sensing technique is among the more promising methods for quality control of polymeric extrusion foaming processes.