A pyrine base, guanine and its corresponding nucleoside, guanosine are tested as eco-friendly green corrosion inhibitors for mild steel in 0.5 M aqueous H2SO4. Both electrochemical and weight loss techniques are employed for the investigation. Results indicate both the inhibitors exert almost identical inhibition efficiency (around 70% at 3 mM concentration level) with guanosine having slightly greater inhibitory effect. From adsorption isotherm study, types of adsorptions, i.e., physical or chemical has been ascertained. Quantum mechanical study (DFT) allows to indicate the atoms or group of the inhibitor molecules involved in electron donation to the metal or the retro-donation.