A method for determining the concentration of hydrogen ion, organic anionic species and anionic species selected from the group consisting of OH-, CO3?=, HS-, ClO¿3-, SO4=, S2O3=, polysulphide and peroxide in an aqueous sample solution, said method comprising subjecting said solution to near infrared radiation at a wavelength region of wave numbers selected from about 7,000 to 14,000 cm-1 through a solution path length of at least 3 mm to obtain spectral data for said solution; obtaining comparative spectral data for said anionic species at known concentrations in aqueous solutions; and correlating by multivariate calibration the relationships between said spectral data of said sample solution and said comparative spectral data to determine said concentration of said anionic species in said sample solution. The method is of particular value for use with pulp liquor determination and control.