In August 2005, Spain approved the plan for renewable energy sources for the period 2005-2010 (PER), including co-combustion installations. Co-combustion in the PER aims to increase power output by 12 185.3 GW h in 5 years and shows great interest in studies of the combustion of mixtures of fossil and biocombustible fuels. This paper presents studies of the co-combustion of soya oil and diesel for thermal heating. The paper begins with a characterization of soya oil as well as mixtures of this oil, with diesel, as fuels. The combustion of the soya oil mixtures and diesel takes place in an installation, where the pressure of injection as well as the air volume of the burner can be changed. Results, in agreement with Spanish environmental legislation, show a greater efficiency of combustion. Conclusions also show that the use of soya oil and diesel, S-30, and S-40 fuel mixtures, for producing thermal energy in conventional equipment, is feasible.