Ab initio calculations were performed on proton bound formic acid and acetic acid dusters, (RCOOHH+ at various levels of theory. Three types of competitive structures were deduced : Open-chain structures are favored energetically for lower members (n less than or equal to 5) of the duster series, chains terminated with a cyclic dimer unit at one end for intermediate n and chains terminated by cyclic dimer units at both ends are favored for high n (n greater than or equal to 6). The computational results corroborate previously reported experimental results from this laboratory, according to which there is a changeover from monomer evaporation from clusters of low n to dimer evaporation at n greater than or equal to 6 for R = H. Fairly good agreement was obtained between calculated and experimental binding energies.