A new type of plasma instability is described in an electronhyphen;beam ionized discharge. This instability occurs in gas mixtures in which the dissociative attachment rate increases strongly with electric field. It has been observed experimentally in He:H2O 740:20 and in He:CO21:1 and 9:1 mixtures. The values of the ionization source functionSand of the electric fieldEat which the wave instability occurs are predicted using a Fourier analysis of the linearized kinetic rate equations. This yields the condition for instability asS (1sol;agr;)Edgr;bgr;sol;dgr;E) minus; bgr;2, where agr; is the electronhyphen;ion recombination coefficient and bgr; is the electron neutral attachment rate.
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