The cavity enhanced absorption (CEA) technique is extended into the 10 mum region using a line-tunable continuous wave CO2 laser. Part of the laser beam is deflected by an acousto-optical modulator (AOM), and is used to excite a mechanically unstable high-finesse optical cavity. In order to assure a stable and optimal transmittance of light through the cavity, the laser frequency and the cavity eigenfrequencies are modulated independently. The time-integrated intensity of the light exiting the cavity, which is inversely proportional to the cavity losses, is measured using a lock-in detection scheme. An absorption detection sensitivity of 1.5 x 10(-6) cm(-1) Hz(-1/2) is readily obtained with a rather simple setup.