Aerosol emissions from aircraft engines contribute to the formation of contrails, which have a climate impact as important as that of aviation’s CO₂ emissions. For the first time, we experimentally investigate the freezing behaviour of water droplets formed on jet lubrication oil aerosol. We show that they can activate to form water droplets and discuss their potential impact on contrail formation. Our study has implications for contrails produced by future aircraft engine and fuel technologies.