A beta 40 and A beta 42 are peptides that adopt similar random-coil structures in solution. A beta 42, however, is significantly more neurotoxic than A beta 40 and forms amyloid fibrils much more rapidly than A beta 40. Here, mass spectrometry and ion mobility spectrometry are used to investigate a mixture of A beta 40 and A beta 42. The mass spectrum for the mixed solution shows the presence of a heterooligomer composed of equal parts of A beta 40 and A beta 42. Ion mobility results indicate that this mixed species comprises an oligomer distribution extending to tetramers. A beta 40 atone produces such a distribution, whereas A beta 42 alone produces oligomers as large as dodecamers. This indicates that A beta 40 inhibits A beta 42 oligomerization.