The mechanical and failure behaviour of basalt fiber (BF) mat-reinforced (30 wt%) composites with vinylester (VE) and vinylester/epoxy (VE/EP) hybrid resins were studied as a function of resin hybridization (VE/EP = 3/1, 1/1 and 1/3) and BF surface treatment. BF was treated either with vinyl or epoxy functionalized organosilanes (VS and ES, respectively). The VE/EP hybrids exhibited an interpenetrating network (IPN) structure in the studied composition range. Specimens, cut of plaques produced by resin transfer molding (RTM), were subjected to static (tensile, flexural) and dynamic (instrumented Charpy and falling weight impact) loading. The fracture toughness was determined under both static and dynamic conditions. The development of the damage zone and its propagation were followed by location of the acoustic emission (AE). It was found that the mechanical properties of the composites were strongly improved when mats with treated BF surface were incorporated. This was mostly traced to the good interfacial adhesion between the BF and matrix according to fractographic inspection. The formation of the interphase (ca. 2 mu m thick) was influenced by the BF treatment: VS coating of the BF resulted in VE-, whereas ES-treatment in EP-enrichment in the interphase due to which the IPN structure also changed locally. This was demonstrated by nanoindentation measurements performed with an atomic force microscopy (AFM) on ion-ablated polished surface specimens. (c) 2005 Springer Science+ Business Media, Inc.