This study uses data for the period 2001-2008 compiled by Taiwan's Environmental Protection Administration (EPA) to examine the environmental Kuznets curve (EKC) relationship between mobile source pollution and air quality in Taiwan. The study employs a regression model which uses the emission volume for three air pollutants-SO(subscript x), NO(subscript x) and CO-as the environmental quality dependent variable, with average annual household income in individual counties and cities as the income indicator, and with independent variables that include a public expenditure variable, the number of vehicles in use, population, and fixed source pollution. The results show that air quality in Taiwan displays the EKC phenomenon, with a significant positive correlation between mobile source pollution (mostly deriving from motor vehicles) and the emission volume for the three types of air pollutants. The data provides further that motorcycles and mopeds are the main mobile source of SO(subscript x) pollution, while cars remain the major mobile source of NO(subscript x) pollution. One point worth noting is that the heavy emphasis on infrastructure projects in local government expenditure has not only failed to help improve the air pollution situation, it has actually led to an increase in carbon emissions.