The reported work is highly related to the DADSS (driver alcohol detection system for safety, ) program, and other related initiatives aiming at the prevention of drunk driving. The possibility of breath alcohol estimation in highly diluted breath samples in a vehicle cabin by using carbon dioxide as a tracer gas to compensate for the dilution has been demonstrated and evaluated elsewhere [4-12]. The infrared sensor technology developed by SenseAir AB, Sweden, is enabling unprecedented sensor performance . However, passive breath alcohol detection requiring no cooperation from the driver has remained a major technological challenge. The aim of the present investigation is to obtain experimental proof-ofprinciple of completely passive, in-vehicle estimation of breath alcohol concentration. A prototype sensor system has been integrated with the casing of the upper steering column within a vehicle. Human subjects, some of them intoxicated by alcohol, are instructed to enter the vehicle and perform a simulated driving task while breathing normally. Sensor signals corresponding to alcohol and CO2 concentration at the sensor position are recorded and analyzed off-line. The sensor CO2 signal pattern includes peaks corresponding to increasing CO2 concentration in expired air reaching the sensor position after leaving the subject’s mouth or nose. These peaks will coincide with peaks in the alcohol signal from an intoxicated subject. From the peak magnitudes an algorithm for breath alcohol estimation has been devised. The results indicate that peaks from normal breathing are readily detectable and quantifiable by the sensors, although the dilution factor DF (ratio between expired and actual concentration measured by the sensor) may be as high as several hundred at the steering column sensor position.