Hydrothermal CO2 reduction using Zn as reductant was investigated. This process has the advantage of avoiding the use of hydrogen with all its safety and environmental concerns, and allows an easy integration with CO2 capture as NaHCO3. In this work, this reaction was studied in batch reactors at temperatures from 275 to 325 degrees C. Conversions up to 60% were obtained with 100% selectivity towards formic acid, at reaction times between 10 and 180 min. A mathematical model correlated with data from literature was developed and is able to correctly predict both experimental and literature data with an averaged error of 3.5%. Main variables of the process were analyzed: temperature, Zn/HCO3- ratio, heating rate, Zn particle size, pressure, etc. The optimum reaction conditions found were 300 degrees C with a rapid heating, and particle sizes of 10 mu m. Zn excess dramatically improves the yield, but a lower excess can be compensated at high pressures.