The paper presents mass and enthalpy balances in the combustion of biomass in stoves. The first part of the paper concerns the biomass as an alternative source of energy, the combustion processes, formation of emissions and the method of calculation of mass and enthalpy balances. In the second part, experimental data are presented involving four types of fuels: foliate pellets, mixed pellets, "American" pellets and sludge pellets of the bituminous coal type. The efficiency of the stove was calculated for all the fuels using mass and enthalpy balances. In addition, analysis of products of imperfect combustion, such as C_xH_y, volatile organic compounds, persistent organic pollutants (polycyclic aromatic hydrocarbons, polychlorinated biphenyls, polychlorinated dibenzodioxines and dibenzofurans) was performed.