This paper analyzes the impact of urbanization on CO2 emissions in developing countries. In this study we treat population as a predictor in the model, instead of assuming a unitary elasticity of emissions with respect to population growth. We contribute to the existing literature by examining the effect of urbanization, taking into account the presence of heterogeneity in the sample of countries and testing for the stability of the estimated elasticities over time. The sample covers the period from 1975 through 2005 for different groups of countries, classified according to their income levels. Our results show that, whereas the impact of population growth on emissions is above unity and only slightly different for upper, middle, and low-income countries, additional demographic variables, namely, urbanization, demonstrate a very different impact on emissions for low and lower-middle-income countries and upper-middle income countries. For the first set of countries, the elasticity, emission-urbanization, is higher than unity, whereas in the second group, the elasticity is 0.72, which is in accordance with the higher environmental impact observed in less developed regions. However, in upper-middle income countries and highly developed countries, the elasticity, emission-urbanization, is negative. The heterogeneous impact of urbanization on CO2 emissions should therefore be taken into account in future discussions of climate change policies.