Effects of inhibitor mixtures (TVE-3A, TVE-3B and TVE-3C) containing formaldehyde in combination with phenol or cresol on corrosion behaviour of N80 steel in 15% HCl solution were investigated using weight loss and potentiostatic polarization technique. The influence of temperature (30-115 degrees C) and lime interval (6-24 h) in the absence and presence of inhibitors on corrosion of the steel in the acid has been studied. TVE-3B has shown the maximum inhibition of 68.6% at ambient temperature whereas maximum inhibition shown by TVE-3A and TVE-3C was found to be 62.2% and 65.7% respectively. The inhibition efficiency was found to gradually decrease with increase in temperature in the case of TVE-3A while TVE-3B and TVE-3C showed a further increase in the protection at higher temperatures (about 81% at 115 degrees C). The protective efficiency of TVE-3B and TVE-3C is comparable with Grade-III (about 88% at 115 degrees C) commercial inhibitor. Inhibition of corrosion is due to adsorption of inhibitors on the metal surface. The adsorption is found to obey Frumkin isotherm. FTIR study of the metal surface products after the corrosion inhibition tests reveal presence of non-linear poly phenyl species, no free carbonyl group indicating formation of a cross linked and three dimensional networking resulting in novalac resin or bakelite by condensation polymerization (acid catalyzed). Oxygen atom, (of -OH group in phenol or substituted phenol) may coordinate to Fe2+ on the metal surface resulting in a protective inhibitor layer. Thermal stability of the inhibitor mixtures was determined by DSC, which has shown that the compounds present in TVE-3A vaporize with increase in temperature while the compounds present in the case of TVE-3B and TVE-3C are thermally stable up to 200 degrees C.