This paper presents a simple way to extend the repressor variables in parti
al least-squares (PLS). The method is particularly suited for nonlinear abs
orption spectra or any other measurements of the soft saturation type. Abso
rption below a certain limit is described as linear and above this limit as
nonlinear. The extension of the regressor variables consists of the absorb
ances above the linear range. The proposed method is compared with standard
PLS. The proposed method gives more parsimonious models and better predict
ions than standard PLS for the two data sets used in this study, ultraviole
t-visible with absorbances in the region 0-2.9.