The wastewater samples, which were obtained from three different ways (with polyester fabric, without polyester fabric and synthetic wastewater), were used for COD and TOC measurements. The values of the COD and TOC in wastewater from disperse dyeing with polyester fabric were lower than the wastewater from disperse dyeing without polyester fabric and synthetic wastewater showing that the dyes in dye-bath were mostly bounded to the fabrics. Quantification of disperse dyes was performed by HPLC, as a selective method after pre-concentration using SPE procedure. The use of multivariate calibration for estimating the concentration of dye mixtures by UV-vis spectrophotometry, as illustrated by C.I. Disperse Blue 79, C.I. Disperse Blue 183, C.I. Disperse Red 82, C.I. Disperse Red 65, C.I. Disperse Yellow 211 and C.I. Disperse Orange 25, recorded at five concentration levels, is described. The importance of calibration design was investigated by calculating the prediction and validation errors and by graphical representation of loadings. The influences of using independent validation sets were emphasized. Calibration design is shown to have major effect on PCR and PLS errors. (c) 2006 Elsevier Ltd. All rights reserved.