A novel method for determination of the compatibility of dyes in mixtures based on the application of principal component analysis is presented. The well known dip-test method is used to dye samples in different binary combinations of cationic dyestuffs. The spectral reflectance of different samples of each mixture that dyed with a given set of dyestuffs by dip-test method has been measured and the corresponding K/S values cure calculated. The actual dimensional properties of each mixture are evaluated by using principal component analysis technique and determination of cumulative percentage variance of the eigenvalues of proposed datasets. Ideally, the K/S spectral data of fully compatible pairs scatter around one dimension, while proportional to the degree of incompatibility of dyes in the mixture, other dimensions should be taken into account and cannot be ignored. Strong correlations are found between the calculated percentage variance and the traditional compatibility values of dyes shown by K value for cationic dyestuffs. The validity of suggested technique is also reconfirmed by normalization of spectral K/S data obtained from different dye sets. (C) 2010 Wiley Periodicals, Inc. Col Res Appl, 35, 313-318, 2010; Published online 14 January 2010 in Wiley InterScience (www. interscience.wiley.com). DOI 10.1002/col.20571