Basic properties of glass filled epoxy such as yield behavior and cohesive strength, which are important in understanding toughening mechanism, are studied using compression tests and double notch 4 point bending (DN-4PB) tests. Three different glass reinforcements, large glass spheres, small glass spheres, and glass fibers, were treated by different methods and used at different volume fractions ranging from 10 to 30 vol%. The effect of different surface treatments and moisture exposure on the yield behavior is studied. It was found that in all types of formulation the yield stress decreased after moisture exposure and that the yield stress was dependent on the surface treatment both before and after moisture exposure. No treatment and treatment of glass reinforcements with aminopropyltrimetboxysilane coupling agent resulted in relatively higher yield stress. Close observation of microstructure by transmission OM showed that the degree of debonding in the specimens for compression tests was quite dependent on the surface treatment after moisture exposure. The decrease in cohesive strength of the neat specimens were observed after moisture exposure. (C) 2003 Elsevier Science Ltd. All rights reserved.