► A novel GC method predicting the roasted coffee composition (Arabica/Robusta) of blends based on two components. ► The method was validated on coffees blends from the Italian market and no statistically significant differences were found. ► The method is useful to ensure the validation of certified roasted coffee blends in routine analysis. ► No need to calculate the response factors, no need to quantify the GC components, no need to add internal standards. Since the price of Arabica is currently more than twice higher than Robusta, a rapid and reliable method for the determination of the roasted coffee blend composition is fundamental for the authentication of commercial blends used for the Italian Espresso coffee. A GC-FID method based on the ratio between the integrated peak areas of kahweol (K) divided by the sum of K and 16-O-methylcafestol (16MCF) was developed. No internal/external standard was used. Moreover, the quantitation of the unsaponifiable compounds is not necessary, as well as the calculation of any response factors. The percentage of Robusta in 34 samples of coffee blends with known composition, and in 48 samples of pure varieties was used to build a cubic polynomial function with R2=0.998. The roasting conditions did not affect the results. Considering eight commercial blends (ranging 0–90% Robusta), no significant difference (two-tailed P=0.817) was registered between the claimed and the predicted composition.