Factors affecting the results of carbon dioxide (CO2) concentration measurements and methods of their automatic compensation are considered for capnometers of two types: lateral flow (LF) and main flow (MF) capnometers. Advantages of the use of one-channel photometer in LF capnometer with automatic calibration were demonstrated. Compensation for temperature, pressure, and presence of concomitant gases and vapors in inhalation mixture is implemented automatically by introducing correction factors in CO2 concentration value. Calibration curves could be obtained empirically for specific capnometer model integrated into the gas analyzer. Carbon dioxide concentration values measured with LF and MF capnometers should be reduced to standard physical conditions STPD and BTPS conditions, respectively (body temperature and pressure, saturation with water vapors).