This study aimed to investigate the corrosion effect of water-in-biodiesel diesel emulsion fuel (W5B5) prepared using three different emulsifiers towards copper. Copper was immersed in W5B5 for 500 h at 25 �. For comparison, additional copper coupons were immersed in diesel fuel and biodiesel fuel for 500 h at 25 �. The W5B5 consists of 2 vol of surfactant, 5 vol of water and 93 vol of biodiesel diesel fuel blend. The utilized biodiesel diesel fuel blend was B5, which consists of 5 vol of biodiesel fuel with 95 vol of diesel fuel. The evaluated emulsifiers were polyglycerol polyricinoleate (PGPR), Xanthan gum, as well as combined Span 80/Tween 80 of equal concentration. Among the performed analysis were mass loss, sedimentation index and total acid number (TAN). Highest corrosion rate was measured for copper exposed to W5B5 prepared with combined Span80/Tween80 followed with W5B5 prepared with PGPR, W5B5 prepared with Xanthan, biodiesel fuel and finally diesel fuel at 0.0011, 0.0009, 0.0007, 0.0006 and 0.0003 mm/yr, respectively. The trend of corrosion rate corroborated with the TAN value. To benefit from the possible reduction in exhaust emission of using water-in-diesel emulsion fuel or water-in-biodiesel diesel emulsion fuel, more studies focusing on formulation of stable emulsion fuel using less acidic emulsifiers and its effect towards compatibility with fuel system metals are required. © Institute of Technology PETRONAS Sdn Bhd (Universiti Teknologi PETRONAS) 2024.