o-Methyl-p-methoxyphenyldithiophosphonic acid (MTPA), o-ethyl-p-methoxyphenyl-dithiophosphonic acid (ETPA) and o-isopropyl-p-methoxy phenyl dithiophosphonic acid (IPTPA) ammonium salts were used as inhibitors on steel corrosion in 1M HCl medium. Concentration of the inhibitors were in the used-as inhibitors on steel corrosion in 1M HCl medium. Concentrations of the inhibitors were in the range 10 −3 to 2×10 −2 M, five different temperatures between 298 and 338K were tried; Tafel extrapolation and linear polarisation techniques were employed. Corrosion parameters and adsorption isotherms were determined from current–potential curves. At room temperature, the three compounds were found to decrease the corrosion rate by around 80% at 1×10 −3 M, and 95% at 2×10 −2 M concentration. The inhibition efficiencies weaken slightly at higher concentrations but in any case, more than 75% inhibition is observed at a temperature of 338K with an inhibitor concentration of 2×10 −2 M. Surface coverage degrees indicate that the adsorption pattern is consistent with the Temkin isotherm.