Experiments were conducted using a batch type micro-autoclave over a temperature range of 100-350 degreesC to understand granite and sandstone reactions in hot water in the absence or presence of excess CO2. Our experiments, for one-week duration, show that the dissolution of granite and sandstone, and the deposition of secondary minerals, is enhanced by the presence Of excess CO2. Major element concentrations of the residual solution from the batch autoclave experiments were higher for the rock (granite or sandstone)/H2O/CO2 system, than in the rock (granite or sandstone)/H2O system, but the sample weight loss was lower in the rock/H2O/CO2 system than in the rock/H2O system. Changes in sample weight and CO2-gas volume content concur with the deposition of aluminium silicate and calcium-aluminosilicate secondary minerals, which indicate fixation Of CO2 in the rock. Our results suggest that it may be possible for granite and/or sandstone to 'capture' CO2, at hydrothermal conditions, and that underground disposal may be a feasible solution to reducing atmospheric emission Of CO2.