A numerical method is described to predict retention times and peak widths of a mixture containing components with known identities in capillary gas chromatography. The procedure is based on extracting thermodynamic values (enthalpy and entropy terms) from Kováts retention indices. Next, a numerical procedure is developed that uses these data to calculate retention times and peak widths on any capillary column containing the same stationary phase but with a different phase ratio. The estimations are based on a sound theoretical basis. The predictions can be performed either in the isothermal or temperature-programmed (single- or multi-ramp) mode. In the temperature programs, which cover a broad temperature range, isothermal plateaus are allowed. Errors in the predictions of retention times are generally less than 4%. Prediction of peak widths under the same conditions can be performed with errors of about 10%. An attractive feature of the approach is, that once the thermodynamic values of the solutes of interest are known, future optimizations can be performed without the need to perform experimental input runs. This indicates that the concept can be used for complete off-line simulations and/or optimizations of gas chromatographic separations.