In this research, round hazelnut cultivars (Fosa, Palaz and Tombul) harvested from the Ordu, Giresun and Trabzon regions in 2001 and 2002 were supplied by the Fiskobirlik Company. Changes in L, a and b values of hazelnut varieties during different roasting applications (125-165 degrees C 10-55 min) were used to determine the optimum conditions of the roasting process and develop mathematical equations. As a result of variance analysis, the effects of the roasting process, cultivars and measurement types on colour values (L, a and b) of hazelnuts were found to be statistically significant (p < 0.01). Values of a and b for hazelnut samples showed an increase while L decreased. The regression coefficients (R-2) which described the prediction models for the dependent variables ranged from 0.73 to 0.97. In addition, changes in colour values, particularly a and L values, gave high R-2 and the obtained three-dimensional (3D) nonlinear equations may be utilized to determine the optimum roasting degree based on time and temperature. (c) 2006 Elsevier Ltd. All rights reserved.