Three α-aminophosphonic acids; namely N-Ethyliminodimethylenediphosphonic acid (N-EDMPA), N-Propyliminodimethylenediphosphonic acid (N-PDMPA) and Ethylenediamine-tetrakis(methylenephosphonic acid) (EDTMP), were synthesized by the reaction of various primary amines, paraformaldehyde and phosphorous acid in acidic medium via Moedritzer-Irani reaction. The structures of the cited compounds were confirmed by UV–Vis, FT-IR, 1H NMR, 13C NMR and 31P NMR spectra. The investigated compounds were screened for their antioxidant activity by mean of the free radical scavenging activity using 1,1-Diphenyl-2-Picryl Hydrazyl (DPPH) and Cyclic Voltammetry (CV) methods. The obtained values of the 50% inhibitory concentration (IC50), show that N-PDMPA exhibit greater antioxidant activity and the order of the antioxidant activities of the studied compounds is: EDTMP < N-EDMPA < N-PDMPA. Furthermore, this biological activity is in good correlation with the density functional theory (DFT) calculations.