Electrical conductivity caused by photoelectron emission from ultrafine particles suspended in a gas has been observed; one obtains information on the concentration as well as on the size of the particles. Furthermore, some bulk and surface properties of small particles not in contact with any substrate can be investigated at any temperature via the photoelectric threshold and the yield of photoelectrons. The method is extremely sensitive, i.e., measurements can be made at a very low particle concentration (∼103cm−3) and small particle radii (<50 A˚).