The hydrogenation of CO2 to formic acid, heterogeneously catalysed by Ru/g-Al2O3 shows a correlation with the permittivity of the solvent and the solvatochromic parameters a and beta. The effect of different alkyl amines on the activity were evaluated and the outstanding activity of the combination of ethanol and NEt3 was explained by strong interactions, proved by ATR-IR spectroscopy. Furthermore we demonstrated that the hydrogenation of CO2 into ethyl formiate - in the presence of ethanol - doesn't need any amine.