The GAB equation is usually used to describe the relationship between moisture content and water activity, but also the relationship between moisture content and relative humidity. It can also be used in other situations, for example, concentration on the surface with respect to concentration in its vicinity, rate of some process with respect to concentration etc. Mathematically, it is a hyperbolic function (linear function in the numerator and quadratic function in the denominator) with the potential maximum of independent variable and the possibility of occurrence of inflection points. The basic problem is, however, the need to determine the value of three coefficients that may be correlated with each other and the accuracy of the fit.