Abstract Despite studies have demonstrated the possibilities of using water in diesel emulsion fuel to reduce the harmful exhaust emission, limited studies are available at present on the effect of emulsified diesel towards metal corrosion. This study therefore aims to investigate the corrosion behaviour of copper exposed to emulsified diesel. Copper immersion tests in diesel, biodiesel and emulsified diesel were carried out for 1200 h at 25 °C. Similar immersion test in emulsified diesel was also carried out for 240 h, 480 h, 720 h, 960 h and 1200 h at 25 °C. Two emulsifiers such as Span 80 and Tween 80 were used to prepare emulsion consisting of 5 vol% of water in diesel. Among the performed analysis were mass loss, surface morphology, elemental composition, surface topography, surface roughness, microscopy, Fourier transform infrared spectroscopy, emulsion characterization, density, viscosity, water content and total acid number. 11.7 times and 4.7 times higher corrosion rate occurred to the copper coupon exposed to emulsified diesel as compared to diesel and biodiesel, respectively. 68% reduction in corrosion rate occurred for the copper exposed to emulsified diesel between 240 h and 1200 h of immersion duration. Up to 3.6 μm and 1.7 μm in pit depth and width, respectively, formed on the copper surface exposed to emulsified diesel for 1200 h at 25 °C. The increased acidity of emulsified diesel by 4.5 times as compared to diesel due to the addition of emulsifiers to form stable water in diesel emulsion is suggested to have caused the increased corrosion rate. Neutralization and/or use of less acidic emulsifier are recommended to reduce the corrosive effect of emulsified diesel. More works on this area are required to establish the compatibility present between emulsified diesel and fuel delivery metals.