A smoke monitoring instrument having first and second chambers, each with a respective photodetector means, a light source common to both chambers and positioned to illuminate part of the interior of each chamber, means for conveying a first air sample from the space to be monitored into said part of the first chamber, means for conveying a second air sample from outside the space into said part of the second chamber, each photodetector means being arranged to receive light from the source which is scattered by its respective air sample but not to receive light directly from the source whereby the effects of suspended material in the first sample originating from outside the space to be monitored can be observed or removed from the output of the instrument.