Concerns about the unequal distribution of greenhouse gas emissions attributable to mobility are gaining increasing attention in scholarly analyses as well as in the public policy arena. The factors influencing the emissions of individuals are largely undocumented, but they are assumed to be the same for all, be they low or high emitters. We use a household travel survey conducted in the metropolitan area of Barcelona to differentiate the factors that result in different rates of emission. It shows that the top 10% of emitters produce 49% of total emissions while 'non-daily' emitters make up 38.5% of the sample. We adopt a quantile regression approach, which reveals significant socioeconomic differences between groups of emitters. Gender, income and home-municipality type are influential in accounting for CO2 emissions for all groups. Educational level appears to be less significant, and occupation shows no significance at all. The study confirms the ineffective nature of toll policy design in the area. Overall, socioeconomic factors have different impacts on different emitting groups, but these characteristics do not impact equally across all the population. Quantile regression using mobility survey data gathered from various cities would provide useful evidence for improving the design of urban mobility policies. (C) 2017 Elsevier B.V. All rights reserved.