Ionization potentials (IPs) of [(CH3)(2)NH](m)(NH3)(n)-H hypervalent radical clusters produced by an ArF excimer laser photolysis of dimethylamine (DMA)-ammonia mixed clusters are determined by the photoionization threshold measurements. The IPs of the DMA(1)(NH3)(n)-H hypervalent radicals decrease rapidly with the number of ammonia up to n = 4, and then its decrease rate becomes much slower for n >= 5. This trend is very similar to that found for NH4(NH3)(n) clusters. The calculated results on the stable structures and IP as well as the observed IP for DMA(1)(NH3)(n)-H indicate that the hydrogen atom-localized site is the NH3 moiety for n = 1, while the doubly coordinated DMA-H is favorable for n = 2-4, and then 4-fold-coordinated NH4 is again more stable for n >= 5. These changes are consistent with the results on the femtosecond pump-probe experiments of DMA(n)-H clusters, Switching of the hydrogen atom-localized site is ascribed to the instability of DMA-H against a hydrogen-atom dissociation.