An experimental investigation has been carried out on the fracture toughness of three thixotropic and room temperature cured epoxy-based adhesives formulated specifically for in situ timber bonding namely CB10TSS (standard adhesive), Albipox is a modification of CB10TSS with the addition of nano-dispersed carboxyl-terminated butadiene acrylonitrile (CTBN) and Timberset is a commercially used adhesive formulation containing quartz, mica and bentonite and cured with an aliphatic diamine (TMD). The fracture toughness behaviour of the adhesives was investigated using the Charpy impact test on unnotched specimens conditioned at 20 C/65%RH and 30 C/95%RH to observe the effect of temperature and moisture. The Charpy impact tests were also performed on notched specimens to evaluate notch sensitivity, and a single-edge notched beam (SENB) test was performed to evaluate the stress intensity factor, KIC. The fracture surfaces were investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Under high impact rate at 20 C/65%RH, toughness was in the order of CB10TSS, Albino and Timber set. CB10TSS and Albino were found to be ductile in the unmatched state and brittle when notched. After exposure to 30 C/95%RH, Albino was tougher than CB10TSS due to an increase in ductility, which allowed the rubber particles to cogitate. Timber set was brittle in both unmatched and notched states. Under low strain rate (SENB) conditions the addition of CTBN significantly improved the fracture toughness of Albino compared with CB10TSS and Timber set. Examination of the topography of the fractured surface revealed marked changes in crack propagation due to the addition of nana- or micro-fillers accounting for the variation in toughness properties. The toughness of thixotropic adhesives, specified for bonding in connections in timber structures, could therefore be enhanced by the inclusion in the epoxy matrix of phase separated liquid rubber.