The aim of this work is to analyze the dielectric manifestation of the glass transition (R mode) of amorphous dendritic macromolecules as a function of architectural complexity. A set of phosphorus-containing dendrimers involving six generations (g ) 0-5) has been investigated by thermostimulated currents and broadband dielectric spectroscopy. The dielectric complex permittivity has been obtained as a function of temperature, in a broad frequency range (10-4-106 Hz). For frequencies higher than 1 Hz, the dendrimers show a high temperature R mode with a Vogel-Tamman-Fulcher type temperature dependence. For frequencies lower than 1 Hz, the R mode takes place in the vitreous state; it has been associated with a series of thermally activated cooperative movements free from generation influence. Dielectric and thermal properties are discussed as regards of fractal architecture and chemical modification of end groups.