The diversity of organic fluids and performance criteria has posed a great challenge for engineers to select and evaluate working fluids for organic Rankine cycles (ORCs). To overcome this challenge, this paper reports on second-law-based analytical expressions for thermal efficiency and specific turbine volume (STV), which reveal key dimensionless parameters relating to operating temperatures and thermodynamic properties of organic fluids. A systematic screen was performed to examine more than 70 working fluids involving a wide range of physical and chemical properties. The working-fluid screen confirmed that STV generally increases with critical temperature, reduced ideal gas heat capacity, and the atom number of working fluids. Different-family fluids may lead to ten-fold relative differences in STV even though they have the same atom numbers (e.g., about 200 m(3)/MJ and 20 m(3)/MJ for benzenes and Alkanes, respectively). The thermal efficiency appears to be independent of critical temperature, or at most, a weak function of critical temperature for low boiling-point wet fluids and other halocarbons, partly due to Jacob number that is independent of or depends weakly on the critical temperature of organic fluids such as alkanes, alkenes, and benzenes.