Phenolic syntactic foams were prepared with phenol-formaldehyde resin (PF), p-toluene sulfonic acid, gamma-aminopropyltriethoxysilane (APTES), and glutaraldehyde (GA) cooperative treated hollow glass microsphere (HGM). Five types of syntactic foams were fabricated by filling different content of modified HGM. Compressive, flexural and thermal properties of the foams were investigated with respect to HGM content and treated temperature. Thermogravimetric analysis (TGA) studies presented a considerable reduction in the weight loss rate of system with the addition of HGM, the residual weight of syntactic foam with 50 vol% HGM at 800 degrees C is 72.45%. Compression and bending tests results showed an increase on compressive and flexural properties for syntactic foams treated at high temperature with increasing HGM volume fraction. The thermal conductivity of all syntactic foams increased with temperature while decreased with introducing HGM, and thermal conduction through solid phase was the main heat transfer approach. The incorporation of HGM was beneficial to improving the thermal insulation property of syntactic foams especially at high temperatures. POLYM. ENG. SCI., 58:2200-2209, 2018. (c) 2018 Society of Plastics Engineers