Two optimal control strategies for carbon dioxide (CO2) enrichment in greenhouse tomato crops have been developed. One uses pure CO2 from a storage tank and the other uses CO2 contained in the exhaust gases of boilers burning natural gas. The optimal strategies maximize the financial margin between crop value and the combined costs of the CO2 used for enrichment and the natural gas used for heating. In this paper, the strategy for optimal control using pure CO2 is presented and compared with strategies used by growers. The optimal strategy for enrichment with exhaust gas derived CO2 is presented in an accompanying paper. Simulations show that at a cost of £0.09 kg-1 for pure CO2 and £0.10 m-3 for natural gas, the optimal enrichment strategy would increase the annual margin of crop value over CO2 and heating costs by £4.6 m-2 (27%) compared to a basic control strategy of enrichment to a concentration of 1000 v.p.m. (parts per million by volume) when ventilators are < 5% open, otherwise enrichment to 350 v.p.m. The optimal CO2 concentration was expressed as an algebraic function of solar radiation, wind speed and ventilator opening angle, and so enabled a quasi-optimal value to be obtained using variables measured by greenhouse environmental controllers. The quasi-optimal equation, with coefficients averaged from simulations over 4 years, gave an increased margin over the basic control strategy of £4.4 m-2 (26%). © 2002 Silsoe Research Institute. Published by Elsevier Science Ltd. All rights reserved.