Some general properties of one-dimensional deflagration waves in a non-conducting inviscid gas at rest are discussed when ionization of the gas takes place across a shock wave which precedes the flame front, and electromagnetic fields are present. The direction of wave propagation, the electric field and magnetic field are taken as a mutually orthogonal triad of vectors. The jump relationships across the gas-ionizing shock wave and magnetogasdynamic combustion wave are investigated and the two Hugoniot curves analysed in detail in the pressure-specific volume plane. The possible types of wave are indicated for arbitrary magnitudes of the upstream electromagnetic field. It is shown that weak gasionizing shock waves cannot exist. For suitably chosen electromagnetic field strenghts the density ratio across the shock wave may be greater than the ordinary gasdynamic limit and, in such cases, the pressure and density ratios are related in an inverse manner, in contrast to the behaviour for ordinary gasdynamic or magnetogasdynamic shock waves. The magnetogasdynamic combustion wave has similar properties to that in ordinary gasdynamics.