Ten pure amino acid specimens (alanine, arginine, asparagine, glutamine, glutamic acid, isoleucine, leucine, phenylalanine, threonine, and tyrosine) and three binary mixtures (phenylalanine/tyrosine, phenylalanine/arginine, and arginine/tyrosine) were bombarded by 10 key argon gas-cluster ions Ar-1000(+) and the sputter-induced emission of positive secondary ions was investigated by orthogonal time-of-flight secondary ion mass spectrometry (TOF-SIMS). A prolific flux of protonated molecular cluster ions (M-n+H)(+) was observed, with up to n=22 for alanine. In most cases, the cluster yields Y-n were found to follow an overall exponential decay with cluster size n, Y-n proportional to exp(-beta n), although a non-monotonous decrease was found for some clusters. The parameter beta varies between similar to 0.2 and similar to 0.65 for the different amino acids; it seems to scale inversely with the respective sputtering yields of the materials. Generally, the yields of dimers relative to monomers are surprisingly high, amounting to 20-170% whereas those of trimers and tetramers are in the range of 10-90%. In amino acid samples with an (unintentionally) enhanced Na content, Na-cationized cluster (M-n+Na)(+) were found in addition to (M-n+H)(+). For both molecular cluster species the yields exhibit a very similar dependence on the cluster size n. From the binary mixtures, the emission of both homo- and heteromolecular clusters was observed; their size distributions seem to indicate that the two components were not homogeneously distributed within the samples. (C) 2015 Published by Elsevier B.V.