Lightguides bent in a U-shape, known as fibre refractometers, have been used as intrinsic sensing elements for near-infrared (NIR) spectrometry. The spectral sensor response depends on both the refractive index and the absorption of the surrounding medium, owing to the variation of the critical angle of total reflection and the ATR (attenuated total reflection) effect, respectively. A multivariate evaluation, using, e.g., partial least squares, may benefit from this additional information and lead to advantageous applications when both quantities of the medium are changing simultaneously. Due to the intrinsic principle, the sensor shows a high immunity against solid particles in the measuring fluid. As an example, a fast in-line determination of fermentable sugars during the mashing process in breweries is demonstrated using this sensor.