This study investigates moisture uptake and diffusion mechanisms in thermoset particle-filled composites for anchor applications. Two epoxy-based with low inorganic filler content and one vinyl ester-based with high inorganic filler content were studied. Material characterization revealed particle size distribution, particle density, the occurrence of pores, and filler composition. Moisture diffusion was monitored at different temperatures, different relative humidities, and in water immersion for up to 540 days. Based on these results a sorption isotherm, temperature dependencies, and diffusion paths could be determined. A phenomenological hybridisotherm, consisting of a combination of Henry's linear isotherm and Type III isotherm, was applied to describe the effect of different fillers on equilibrium moisture uptake. Fickian diffusion was observed for all composites with temperature-dependent Arrhenius relation. These observations allowed us to suggest three diffusion paths: through the polymer matrix, on the matrix-filler interfaces, and through matrix and fillers simultaneously.