Spray-dried lactose consists of an amorphous component (10–20%) as well as the crystalline monohydrate form [1]. It is commonly used as a diluent in direct compression, mainly because of its better flow characteristics compared to pure crystalline lactose. The amorphous form is metastable and can relative easily crystallize, which will affect the functionality of the pharmaceutical product. It is therefore of interest to establish methods for non-invasive and rapid assessment of the level of crystallinity in a pharmaceutical formulation. In this study, two spectroscopic methods, near infrared (NIR) spectroscopy and terahertz time domain spectroscopy (THz-TDS), are compared for their ability to determine low levels of crystalline lactose in a mixture. The aim was to find the limit of detection and limit of quantification for the two techniques. Partial least squares (PLS) regression models were developed and the root-mean-square-error-of-cross-validation (RMSECV) for the models with full concentration range were found to be 2.91% (w/w) and 0.87% (w/w) for THz-TDS and NIR, respectively. Calibrations developed on samples containing 0–10% (w/w) crystalline material resulted in RMSECVs of 0.30% (w/w) and 0.20% (w/w) for THz-TDS and NIR, respectively, while the limits of detection were 0.80% (w/w) and 0.43% (w/w), respectively. Both instrumental techniques are thus able to quantify the content of crystalline lactose in a mixture. To select one method over the other in an industrial quality assurance setting, further includes other aspects - such as sample handling, sample size, outlier information, instrument stability, etc. In all these aspects, NIR spectroscopy currently performs better than THz-TDS.