In diffusion charging theory, it is assumed that each ion-particle collision leads to the transfer of charge from ion to particle, and that charge transfer will not occur upon collision between a vapor molecule and a charged particle. However, in chemical ionization, charge transfer can occur in two directionsfrom charge-donating ion to vapor molecule and back from charged vapor molecule to the original charge-donating species. Both aerosol diffusion charging and chemical ionization are collision-based charge transfer processes, and for particles only slightly larger than vapor molecules (aerosol clusters), the line between diffusion charging and chemical ionization becomes blurred. We examined the charge transfer from aerosol clusters (positively charged amino acid clusters) in the ∼ 1.0 nm size range to neutral vapor molecules (trimethylamine) at atmospheric pressure by using a combined experimental and theoretical approach. It was found that for singly charged amino acid cluster ions composed of 1, 2, and 3 amino acid molecules, the rate of charge transfer to trimethylamine vapor molecules was clearly observable, particularly for clusters composed of 1 and 2 molecules. The charge transfer rate for singly charged clusters with 4 or more amino acid molecules was consistently close to 0, indicating that the rate of charge transfer from clusters to vapor molecules is size dependent. The charge transfer rates also varied with cluster's chemical composition. Overall, this study demonstrates that small aerosol clusters (0.5 nm) can lose charge through collisions with vapor molecules, which is typically not considered in diffusion charging theories.