Vacancy and antisite defect formation energies in D019-SnTi3 are calculated by an ab initio approach. Based on a statistical-thermodynamic model, the defect concentrations are calculated as function of temperature and deviation from stoichiometry. The constitutional defects are antisite Sn atoms and antisite Ti atoms in Sn-rich and Ti-rich SnTi3, respectively. The dominant thermal defects are composed of two antisites for stoichiometric alloys. For Sn-rich D019-SnTi3 alloys, the thermal defect is an interbranch where one antisite Sn atom is replaced by four Ti vacancies. For Ti-rich D019-SnTi3 alloys, the thermal defect is a five point defect comprising one antisite Ti and four Ti vacancies. The effective defect formation enthalpies are derived for different concentration regions of D019-SnTi3. The Gibbs energy as well as the Sn and Ti chemical potentials in the D019-SnTi3 are obtained as function of composition for various temperatures. The extension of the one phase domain of D019-SnTi3 in the Sn–Ti phase diagram is discussed.