An experimental study on the thermal decomposition of formyl radicals (HCO) under high pressure conditions is presented for the first time. The experimental conditions covered in this study range from 1 to 140 bar of helium and temperatures between 590 and 800 K. Furthermore, the reaction was studied at 700 K and pressures from 1 to 14 bar of nitrogen to obtain information on the kinetics of the reaction under combustion conditions. An analysis of the pressure dependence of the rate constants shows that standard treatments like a Lindemann–Hinshelwood model or a master equation formalism are not able to describe the experimentally observed pressure dependence adequately. An isolated resonance model, however, which is based on calculated resonance lifetimes from H.-M. Keller, H. Floetmann, A. J. Dobbyn, R. Schinke, H.-J. Werner, C. Bauer and P. Rosmus (J. Chem. Phys., 1996, 102, 4983) allows for a description of the observed T- and p-dependence. Based on this model and our experimental data obtained in nitrogen as well as experimental results obtained by R. S. Timonen, E. Ratajczak, D. Gutman and A. F. Wagner (J. Phys. Chem., 1987, 91, 5325) in nitrogen we are able to give a simple expression for the rate constant at temperatures between 500 and 1000 K and pressures between 0.01 and 100 bar: k(T,[M]) = 3.3 × 10⁹ × {[N₂]/(10¹⁹ cm⁻³)}^0.865 × exp{−(70.15 kJ mol⁻¹)/RT} s⁻¹. The uncertainty of the rate constant calculated by this expression is estimated to be 30%.