A method for determining the aluminium content of an aluminium hydroxide suspension using near infrared (NIR) transmittance spectroscopy has been developed. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) was used as reference method. The factors influencing the NIR analysis, such as different sample containers (transmission cell and cuvette), sedimentation in the suspension, day-to-day variation and batch-to-batch variation have been studied before constructing a calibration model. Seven dilutions (0-4100 mg Al/L) of five batches of aluminium hydroxide suspension samples were measured by NIR transmission each on five different days, with total of 175 spectra used for the calibration set. The multivariate data analysis technique partial least square regression (PLSR) was applied to build the calibration model. Six batches of aluminium hydroxide samples were used for the test set. ICP-AES and NIR transmittance spectroscopy exhibit comparable precision and accuracy. The NIR method provides several advantages: no complicated sample preparation; easy to operate; fast and non-destructive. In conclusion, NIR transmittance spectroscopy can be an alternative analytical method for determining aluminium content in aluminium hydroxide suspension. (c) 2007 Elsevier Ltd. All rights reserved.