A detailed analytical study of the formation of the products of the pyrolysis of n-butane in the temperature range 420 to 530 $^\circ$C and at initial pressures between 10 and 150 mm Hg has been carried out. A free-radical mechanism incorporating that originally proposed by Rice is shown to give an excellent quantitative account of the reactions occurring. Rate parameters for some of the reactions involved have been calculated. The unimolecular decomposition of the ethyl radical is shown to be in its pressure dependent region throughout the range of conditions employed, and the fall-off of the rate constant with pressure is shown to be well described by the classical theory of Kassel. The important role of the ethyl radical in determining the kinetic features of paraffin pyrolyses is outlined.