We have presented experimental and theoretic evidence for the existence of sinter-resistant cluster sizes for Pt 1-8 /Al 2 O 3 . Pt 4 and Pt 7 were predicted to not only be particularly stable against sintering from a monodisperse cluster size starting distribution, but were also shown to persist even when (a) not initially present in the monodisperse cluster size distributions and (b) when sintering starts form a mix of initial cluster sizes. This was validated experimentally using ISS before and after heating to 750 K. The stability of these clusters is attributed to the ensemble energetics of forward vs. reverse sintering. For both Pt 4 and Pt 7 , the energetics of monomer movement to and from other cluster sizes is generally energetically unfavorable. Therefore, even in a system with multiple cluster sizes, which should therefore generally have a strong driving force for sintering, we see Pt 4 and Pt 7 persist in our simu-lations. This stability is furthermore attributable to the particular geometric and electronic structure of the global minimum Pt 4 and Pt 7 isomers, which are much more crystalline than clusters of other sizes, with a greater number of bonds per Pt atom than all other cluster sizes and isomers.