A series of rigid polyimide (PI) foams were synthesized via the reaction of a first solution with a second solution. The first solution was isocyanate-terminated polyimide prepolymers; the second solution contained deionized water and surfactant. The effect of different water contents and isocyanate index on the structures and properties of rigid PI foams were investigated. The apparent density, hardness, compressive strength, and the 5% weight loss temperatures (T5%) decreased with the increase of water content. With the increase of isocyanate index, the apparent density and the T5% values decreased, whereas the glass transition temperatures (Tg) increased and the hardness, compressive strength first increased and then decreased. The rigid PI foams composed of closed-cells were confirmed by scanning electron microscopy. The maximum compressive strength of rigid PI foams prepared was up to 1.31 MPa. Moreover, excellent thermal stability was presented with the T5% values were all above 360 degrees C and the residual weights of the foams (Rw) were more than 50% at 800 degrees C. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013