The low-pressure counting efficiency of three modified condensation particle counters (CPCs, TSI Model 7610) was investigated. In this study, we describe the modifications of the counters, present a "new" low-pressure calibration set-up, and show the resulting CPC counting efficiency curves as function of operating pressure (160-1000 hPa) and temperature difference between the saturator and condenser block (10.5-26.0 degreesC). Furthermore. particle coincidence in the CPC optics was studied. The presented CPCs are able to measure particle number concentrations of up to 40,000 particles/cm(3) at pressures down to 160 hPa. The lower threshold diameter of the CPCs can be varied between 4 and 23 nm.