The characteristics of an air-equivalent ionization chamber used for routine exposure measurements were investigated. The air-equivalent material used was a mixture of 91% polycarbonate [(C16H14O3)n] and 9% E-glass (54% SiO2; 1% Fe2O3; 1% Na2O2+ K2O2; 1% TiO2; 15% CaO; 8% B2O3). This is equivalent to a mixture of commercial thermoplastics consisting of 55% Makrolon-3100 natural and 45% Makrolon-8020 natural (Bayer). The parts of this chamber have been injection moulded and the chamber had volume of 1l and wall thickness of 2 mm. The inner surfaces of the chamber were coated with a conductive air-equivalent layer. The response of the chamber was slightly gamma-energy dependent in the 0.04 MeV to 1.25 MeV energy range.