SP Fire Research has recently conducted a research project which demonstrated that CO sensors may be more suitable than photoelectric detectors for detecting smouldering fires at an early stage. This was done by experimentally comparing photoelectric detectors with CO sensor in simulated bedroom fires. The response times of the CO sensors were significantly faster than for the photoelectric detectors. Furthermore, whereas the levels of fire gases at the time of CO alarm activation were sub-toxic, the results indicate that the CO dose may exceed critical values before photoelectric detectors activated.