A general mathematical model is presented for predicting the concentrations of chemically reactive compounds in indoor air. The model accounts for the effects of ventilation, filtration, heterogeneous removal, direct emission, and photolytic and thermal reactions. The model is applied to the induction of photochemically reactive pollutants in a museum gallery, and the predicted NO, NO_x, and O₃ concentrations are compared to measured data. The model predicts substantial production of several species due to chemical reaction, including HNO₃, HNO₂, NO₃, and N₂O₅. Circumstances in which homogeneous chemistry may assume particular importance are identified, and include buildings with glass walls, indoor combustion sources, and direct emission of olefins. The model as presently formulated may be used to assess effects of filtration of selected compounds, to design indoor air quality control strategies based on ventilation scheduling, and so on. However, the ventilation-air conditioning-heating engineer dealing e. g. with museums, will hardly be able to make full use of the model without the co-operation of a specialist in air chemistry. -- AATA