A congestible urban transport system is considered in which cars and buses are used for commuting and non-commuting trips. Commuting is a strict complement to taxable labour supply. The optimal tax structure for raising a given amount of government revenue is examined for the cases where differentiation of transport tolls between trip purposes is and is not possible. An application to Belgian urban environments shows that optimal toll differentiation produces significant efficiency improvements. Without differentiation, reforming transport taxes generates substantial gains only when the labour tax can be reduced. Copyright The editors of the "Scandinavian Journal of Economics", 2003 .