The aim of this work was to evaluate if the addition of the chemical absorbents diethanolamine and potassium carbonate affects the CO2 biofixation, growth and biomass composition of Spirulina sp. LEB 18. The association of the diethanolamine (DEA) and potassium carbonate (K2CO3) absorbents increased the dissolved inorganic carbon concentration in the cultivation medium, allowing greater CO2 biofixation by the Spirulina. Higher biomass concentration (2.1 g L-1) and maximum productivity (174.2 mg L-1 d(-1)) were observed with the mixture of 1.64 mmol L-1 of DEA and 0.41 mmol L-1 of K2CO3. In this cultivation condition, Spirulina sp. LEB 18 showed high protein content (58.8 w w(-1)) and an increased carbohydrate concentration (23.7% w w(-1)). The addition of these absorbent concentrations may be applied in the cultivation of Spirulina sp. LEB 18 to increase CO2 biofixation and cell growth.