The thermally activated unimolecular dissociation of some unsolvated complexes of helical and globular alanine/glycine-based peptides have been studied by electrospray mass spectrometry and ion mobility measurements. Rate constants have been measured as a function of temperature, and activation energies and entropies have been determined for dissociation of the Ac-(GA)(7)K.Ac-A(GA)(7)K + 2H(+) (Ac = acetyl, G = glycine, A = alanine, and K = lysine) dimer (dominant conformation assigned to a V-shaped helical dimer) and the 2Ac-(GA)(7)K.Ac-A(GA)(7)K + 3H(+) and Ac-(GA)(7)K.2Ac-A(GA)(7)K + 3H(+) trimers (dominant conformation assigned to a pinwheel-shaped helical trimer). The activation energies were found to be 91 kJ mol(-1) for the dissociation of the dimer, and 72 and 77 kJ mol(-1) for dissociation of the trimers. A key advantage of the experimental approach described here is that it permits the conformations of the dissociating peptides to be observed. For the Ac-K(GA)(7).Ac-KA(GA)(7) + 2H(+) dimer (which has the lysines at the N-terminus), there are two major conformations present (assigned to a compact globular aggregate and an antiparallel helical dimer) which appear to dissociate with different rates. The Ac-K(GA)(7).Ac-KA(GA)(7) + 2H(+) dimers dissociate at a significantly higher temperature than the V-shaped Ac-(GA)(7)K.Ac-A(GA)(7)K + 2H(+) dimer. The alanine analogue of the antiparallel helical dimer, Ac-KA(14).Ac-KA(15) + 2H(+), does not dissociate significantly at the highest temperature accessible (similar to423 K).