One of the most important parameters to quantify an aerosol is the particle number concentration. Condensation particle counters (CPCs) are commonly used to measure the aerosol number concentration in the nanometer size range. To compare the data from different measurement stations and campaigns, it is important to harmonize the instrument specifications, which is why the CEN/TS 16976:2016 technical specification was introduced for CPCs. Therein, the parameters of the CEN CPC are specified for standard pressure and temperature. However, CEN CPCs are used in various surroundings, on high mountains or on airplanes, where they are exposed to low-pressure conditions. Here, we present the pressure-dependent performance (including the concentration linearity and counting efficiency) of two different models of CEN CPCs, the Grimm 5410 CEN and the TSI 3772 CEN. We found that their performance at 1000 and 750 hPa was in accordance with the CEN technical specifications. Below 500 hPa, the performance decreased for both CPC models, but the decrease was different for the two models. To gain insight into the performance of the two CPC models, we performed a simulation study. This study included simulations of the saturation profiles and calculations of internal particle losses within the CPCs. The simulations reproduced the overall performance decrease with decreasing pressure and reveal that the internal structure of the CPC has a significant influence on the performance. We anticipate our publication to provide a deeper understanding of the counting efficiency of CPCs and their pressure dependence. Our findings might be a starting point for new standards that include the pressure-dependent performance, or they could help in designing new CPCs.