The stability of a negatively chargedpolyethylene-structured [GaH2]nn−cluster ion was inves-tigated by varying the K+/Rb+ratio in (KxRb1−x)n[GaH2]n(0≤x≤1). Neutron, X-ray, and IR spectroscopies wereused to characterize the new phases. Between the limitingcompositions Kn[GaH2]nand Rbn[GaH2]n, the [GaH2]nn−chains remained almost identical, indicating a stable specie.For rubidium-rich samples up to a potassium contentcorresponding to (K0.5Rb0.5)n[GaH2]n, two phases coexistin the samples, RbGaH2and (K0.5Rb0.5)n[GaH2]n, with aratio mirroring the relative alkali-ion content. The twophases have a different alignment of the [GaH2]nn−chains.For potassium-rich samples beyond (K0.5Rb0.5)n[GaH2]n,the samples were single-phased. The unit cell volume ofthe new (K0.5Rb0.5)n[GaH2]nstructure type shrinksaccording to Vegard’s law as smaller K+ions substitutefor larger Rb+ions. The [GaH2]nn−chains remained,however, almost identical. IR spectra from the differentphases were very similar, exhibited stretching, scissoring,and rocking modes similar to those in ordinary poly-ethylene, but shifted to lower frequencies, reflectingweaker Ga−H bonds. The existence of stable Kn[GaH2]nand Rbn[GaH2]n, would help to dehydrogenate KGaH4and RbGaH4upon heating. If this could be transferred tolighter alanates and borohydrides, it could be possible todevelop more functional hydrogen-storage systems.