Thermodynamic analysis of the chemical vapor deposition (CVD) of P3N5 has been performed using the method of the minimization of the Gibbs' free energy in order to evaluate the PH3-NH3-N-2, PC3-NH3-H-2 and PBr3-NH3-H-2 gaseous mixtures for their potential to synthesize single-phase P3N5 films at high yields. The conditions for the deposition of P3N5 have been determined as a function of input reactant gas ratio of PX(3)/(PX(3)+NH3) (X=H or Cl or Br) and deposition temperature at atmospheric pressure. A single phase P3N5 is deposited at almost all reactant ratios and at temperatures below about 700 K when the PH3-NH3-N-2 system is used. The use of halide gas mixtures limits the formation of single phase P3N5 to narrow regions of temperature and input reactant gas ratio. The gaseous species generally present in greatest abundance are H-2, N-2, NH3, PH3, HCl, P-4, PCl3, P-2, HBr, PBr3 and PN. The thermodynamic analysis suggests that among the systems investigated here, the PH3-NH3-N-2 mixture is the most promising because simultaneously it gives the highest P3N5 deposition yield and allows better control of the CVD process for the synthesis of P3N5 films.