A modular unit for applying opening devices to packages of pourable food products includes a first conveyor for feeding the opening devices successively along a first path, a second conveyor for feeding the packages successively along a second path, a transfer mechanism for transferring the opening devices along a third path from a pickup station located along the first path, to an application station for applying the opening devices to respective packages and located along the second path, and a processing device for performing specific operations on the opening devices prior to application of the opening devices to the respective packages. The transfer mechanism defines a base module of the unit, and the processing device includes different types of processing devices forming part of different auxiliary modules selectively connectable to the base module to define different units for applying the opening devices to respective packages.