The Charpy impact test has been employed to study the fracture toughness and failure mechanisms of E-glass woven fabric reinforced vinyl ester matrix composites with five different types of fibre surface treatment. The results revealed significant dependency of the impact fracture toughness values on silane coupling agents and hygrothermal ageing. Failure mechanisms were characterized using a scanning electron microscope, and correlated with the measured fracture toughness, temperature and water absorption.