Suitable forms of the equations for the flow of an inviscid, non-heat- conducting gas in which chemical reactions are occurring are derived, The effects of mass diffusion and non-equilibrium amongst the internal modes of the molecules are neglected. Special attention is given to the speeds of sound in such a gas mixture and a general expression for the ratio of frozen to equilibrium sound speeds is deduced. An example is given for the ideal dissociating gas. The significance of the velocity defined by the ratio of the convective derivatives of pressure and density is explained. It is the velocity which exists at the throat of a convergent-divergent duct under maximum mass flow conditions, and it is shown that this velocity depends on the nozzle geometry as well as on the 'reservoir' conditions. As an illustration the phenomena of sound absorption and dispersion are discussed for the ideal dissociating gas. The results can be concisely expressed in terms of the frozen and equilibrium sound speeds, the frequency of the (harmonic) sound vibration and a characteristic time for the rate of progress of the reaction.