A description of a portable photoionization detector (PID) which has been developed by Photovac Incorporated. The instrument (Photovac model 10A10) is capable of accepting air samples directly, is sensitrve enough to detect a variety of contaminants at concentrations of parts per billion and below, and is fully portable. The PID is combined with a portable gas chromatographer so that air samples of up to 1 mL in volume may be analyzed at the sample site for contaminants at sub-parts per billion levels. This moderately priced instrument has wide applications for monitoring the museum environment. -- AATA