Different names/concepts and therefore different spatial boundaries for cities in China are responsible for the conflicting and confusing results associated with urban CO2 emissions accounting. In this study, four types of urban boundaries, i.e., city administrative boundary (U131), city district boundary (UB2), city built-up area (UB3) and urban proper (U134), were identified and defined. Tianjin was subsequently selected as the case city to illustrate the different performances of CO2 emissions with respect to these four boundaries using a 1-km grid dataset built bottom-up by point-emission sources. Different urban boundaries can induce a difference in CO2 emissions as large as 654%. UBi and UB2 are not the appropriate proxies for urban boundaries in the analysis of urban CO2 emissions, although UB1 is a widely adopted boundary. UB3 is a good representative of city clusters and urban sprawl in a certain region, whereas UB4 is the appropriate system boundary for such issues as urban CO2 emissions in light of landscape characteristics and pertinent human activities, as well as the comparability to counterparts in developed countries. These results provide sound policy implications for the improvement of urban energy management and carbon emission abatement in China. (C) 2013 Elsevier Ltd. All rights reserved.