The model in which a partial least squares regression algorithm was used worked slightly better than the one based on principal component regression. The errors found for diclofenac sodium determination in testing samples equal 2.2 and 2.6%, respectively. RSEP values for additives varied in the 0.4–2.4% range for the PLS model and in the 0.5–2.1% range for the PCR one.In the course of constructing multivariate models for aminophylline, two spectral ranges were applied: 3570–2587 and 1716–415 cm−1. The calibration curves for theophylline were characterised by R2 values in the range 0.983–0.999 (Fig. 5). The RSEP values obtained for theophylline in the case of the testing sample set equal 2.7% for PLS and 2.3% for PCR methods. A full set of calibration results for the API is collected in Table 2. As one could notice, the RSEP values for the multivariate models were even a few times smaller than those found for the univariate approach for both the studied APIs.