Abstract: A computer implemented method for on-line determination of concentrations in an HF alkylation process catalyst stream containing HF acid, ASO, water and optionally an additive which suppresses the vapor pressure of HF acid. The method employs near-infrared spectroscopy coupled with chemometric data analysis. The data analysis uses a partial least squares regression technique which determines unknown concentrations based on spectral calibration data. In use, an absorbance spectrum of material is obtained, e.g., from a slipstream taken from the process alkylation catalyst stream, and the detected spectrum is mathematically analyzed with the aid of the computer to simultaneously predict concentration of HF acid, ASO, water and optionally the additive which suppresses the vapor pressure of HF acid.