The thermal and mechanical properties of polybenzoxazine thermoset networks containing varying amounts of phenolic Mannich bridges, arylamine Mannich bridges, and methylene bridges have been investigated. In materials based on, m-toluidine and 3,5-xylidine, the onset of thermal degradation is delayed until around 350; degreesC with no significant effect on the final char yield. The first of the three weight-lass events usually seen in. aromatic amine-based polybenzoxazines is absent in these two materials. Materials with additional amounts of arylamine Mannich bridges and methylene bridges show improved mechanical properties, including higher crosslink densities and rubbery plateau moduli. Correlations between the observed mechanical properties and network structures are established.