To implement process analytical technology in beer manufacturing, a systematic study of the ternary system water–maltose–ethanol with respect to the critical process parameters, density, speed of sound and temperature was performed. The results are presented in the form of temperature and mass-fraction-dependent polynomial expressions. On average, a variation of 1% mass fraction maltose results in variations of 3.548 m s−1 ultrasound velocity and 0.0041 g cm−³ density, whereas in the case of ethanol, the variations are 8.060 m s−1 and −0.0018 g cm−3. Indeed, the relations are strictly nonlinear. Nevertheless, the determined data show the feasibility to predict online, concentrations of multicomponent mixtures of polar liquids by determining density and ultrasound velocity. With <0.1% error, the measured data show excellent agreement with reference data of binary mixtures as given in literature.