An aged epoxy prepreg's glass transition temperature (Tg) is determined using Near Infrared Spectroscopy (NIRS). A miniaturized Near Infrared (NIR) spectrometer and a full-scaled FTNIR spectrometer are used for Tg determination and different measuring setups are tested for the miniaturized spectrometer. Partial least square (PLS) regression is used to develop calibration models by utilizing different spectral pretreatments. It is shown that with the miniaturized spectrometer and a suitable measurement setup, PLS model qualities similar to those of the FTNIR spectrometer can be achieved in cross-validation (CV), with a determination coefficient of CV (R ²CV) of about 0.97 and a root mean square error of cross-validation (RMSECV) of approx. 0.7 °C in the measuring range between 25 °C and 42 °C. Cost-effective, miniaturized NIR spectrometers are thus a suitable technology for the quality assurance of prepregs.