The response characteristics of four condensation nucleus counters (CNC's) to monodisperse sodium chloride aerosol particles in the 3-50 nm diameter range are compared with concentrations that were measured with an aerosol electrometer. Measurements were conducted in a workshop that was held at the University of Minnesota Particle Technology Laboratory, 24-28 September 1984. The CNC's that were investigated included a variable expansion ratio CNC (Niessner et al., 1984), a commercial steady flow CNC (TSI, Inc., Model 3020), a modified version of the TSI Model 3020 CNC (Stolzenburg and McMurry, 1984), and a mixing type CNC (Kousaka et al., 1982). The latter two instruments were specifically designed for accurate measurements of aerosol concentrations for particles smaller than 10 nm. For particles of 24 and 50 nm, responses of all CNC's were within about ± 10% of values determined with the aerosol electrometer. For particles smaller than 10-20 nm, concentrations measured with the electrometer exceeded those measured by the CNC's. For 3 nm NaCl particles, average CNC counting efficiencies (CNC response/electrometer response) were 0.06 ± 0.03 (TSI, Inc. Model 3020), 0.60 ± 0.18 (Kousaka et al. mixing-type CNC) and 0.83 ± 0.17 (Stolzenburg and McMurry modified Model 3020 CNC). The size-dependent counting efficiency of the variable expansion CNC was similar to that for the TSI CNC. These data indicate that counting efficiencies for very small particles depend substantially on instrument design.