This paper reports petrographic and chemical data and magnetic susceptibility values for trachytes of quarries recognizable within the Euganean Hills and sets out diagnostic parameters for these rocks. The diagnostic scheme proposed is basic for the definition of provenance of trachytes used in ancient artefacts spread over northern Italy. Trachytes were extensively used by the Romans for paving Via Aemilia, the most important road in the Po Plains connecting Ariminum (present-day Rimini on the Adriatic sea) to Placentia (present-day Piacenza on the Po river). Remains of the trachytic flagstones (basoli) used for paving segments of Via Aemilia crossing Regium Lepidi (present-day Reggio Emilia) and Bononia (present-day Bologna) have been characterised petrographically, chemically and also using magnetic susceptibility data. The discriminating scheme proposed here for the Euganean trachytes has been applied to the data obtained for basoli and thus the provenance of each investigated block has been inferred. The rocks sampled at Regium Lepidi mostly come from Monselice (only a few blocks from Monte Oliveto and Monte Merlo) and those of Bononia come mostly from Monte Merlo and, subordinately, from Monte Oliveto and Monselice (only one sample from Monte San Daniele). The dominant provenance of blocks, which is different in the two colonies distant by less than 50 km, suggests that the two segments of Via Aemilia were not built simultaneously.