The influence of weathering on fracture toughness, J(c), yield strength, sigma(y), local ultrasonic velocity, V-R, and microhardness, H-u, in unreinforced and a short glass-fibre-reinforced polyester-based thermoplastic "Xenoy" has been investigated. Unreinforced material weathered for 11 months outdoors in Perth, West Australia, exhibited a significant decrease in V-R and J(c), whereas little change was shown in sigma(y), H-u and the fracture surface morphology. Irradiation for 1000 h by artificial ultraviolet rays upon the unreinforced material caused a considerable increase in H-u and only a slight deterioration in V-R of the surface layer. Filling with the short fibres induced an improvement in sigma(y) and a large reduction in J(c) for the unreinforced material. The natural weathering of the reinforced material caused a small reduction in J(c) but a large degradation in the slope of the R-curve. It was concluded that the measurement of J(c) and the slope of R-curve in combination with V-R and H-u was an effective way to study the effects of weathering on engineering plastics.