Urban buses are logical candidates for catalytic trap oxidisers (CTO's). Smoke and particulates present more acute problems in an inner city environment than on open roads, while on the other hand urban bus operation gives better chances of succesful regular regeneration than long-distance truck operation. With the support of the Dutch Ministry for Environmental Protection two CTO's are tested in urban bus operation in the city of Rotterdam. The program comprises extensive mapping of the exhaust gas quality at the start (gaseous emissions, smoke, particulates, sulfate and aldehydes) and regular monitoring of durability aspects at 30,000 km intervals. This paper presents the results of the first evaluation.