Vapor pressures of five methyl-substituted dicarboxylic acids are inferred from measurements of evaporation rates of sub-micron particles using the TDMA technique at ambient temperatures. Vapor pressures obtained are: log(p(methyl-malonic)(0)/Pa) = -5538 K/T + 15.5, log(p(dimethyl-malonic)(0)/Pa) = -6195 K/T + 17.2, log(p(2-methyl-succinic)(0)/Pa) = -4800 K/T + 13.3, log(p(2,2-dimethyl-succinic)(0)/Pa) = -3515 K/T + 9.0, log(p(3-methyl-glutaric)(0)/Pa) = -3173 K/T + 7.5, log(p(3,3-dimethyl-glutaric)(0)/Pa) = -3138 K/T + 7.6, log(P-3-methyl-adipic(0)/Pa) = -11227 K/T + 33.8. These vapor pressures are compared with available literature data and with vapor pressures of the un-substituted acids. Our results demonstrate that molecular structure is important for solid state vapor pressures of secondary organic aerosol components at ambient conditions. (C) 2004 Elsevier Ltd. All rights reserved.