The in-situ bonding of pultruded fibre-reinforced plastic rods into timber structural members is a commonly used technique for making timber-to-timber connections and for the strengthening and repair of timber structures. Ideally, the adhesive should be thixotropic, shear thinning, room temperature cure, environmentally stable, solvent-free and applied without pressure. This study investigates the moisture absorption characteristics of three adhesives, specially formulated for bonded-in timber connections, where two of the adhesives are modified with nano- or micro-particles. The three adhesives are denoted as CB10TSS (standard adhesive), Albipox (standard adhesive with CTBN rubber additions) and Timberset (standard adhesive filled with ceramic particles). The aim of the additions is to improve the environmental stability of the standard adhesive as well as enhancing mechanical properties and raising the glass transition temperature. The effect of high temperatures and high humidity on the properties of the three adhesives was determined following conditioning at different combinations of temperature and relative humidity (20, 30 and 50 C/95% RH) and soaking in water at 20 C. In all cases the moisture uptake for the rubber-modified adhesive was less than for the standard adhesive, but the ceramic particle-filled adhesive exhibited the lowest moisture uptake overall. Exposure to humid environments at temperatures lower than Tg resulted in water uptake characterized as Fickian, which had only a modest effect on properties. However, exposure to humid environments at temperatures higher than Tg resulted in non-Fickian uptake of water and significant changes to the diffusion and permeability coefficients.