Optimized control strategies for carbon dioxide (CO2) enrichment of greenhouse tomato crops using CO2 from the exhaust gases of boilers burning natural gas are presented. In one group of strategies, the heat produced during CO2 generation which exceeds the immediate heat requirement of the greenhouse is stored as hot water and used subsequently for heating. The simulations show that, use of optimal control can increase the financial margin of crop value over the combined expenditure on gas used for CO2 and heating by £2.3 m-2 (11%) when heat is not stored and by £4.9 m-2 (24%) when heat is stored, compared with enriching with CO2 only when heating is required. A 30% increase in gas price reduced the financial margin by 11%, whereas a 30% increase in tomato price increased the margin by 40%. The capacity of the heat store places a limit on the amount of heat that can be stored and consequently on the amount of natural gas that can be burnt and the associated amount of CO2 produced during the day. The optimum size of heat store is 20 × 10-3 m3 per unit greenhouse area. © 2002 Silsoe Research Institute. Published by Elsevier Science Ltd. All rights reserved.