Simultaneous interpenetrating polymer networks (SINs) were prepared from nadic methyl anhydride-cured epoxy resin and bismaleimide (BMI) of which the composition ratios were, respectively, comprised of 3/1 (EM-25 series) and 1/1 (EM-50 series), under different gelling conditions for both resins. Dynamic mechanical and tensile properties were measured for these SINs. Turbidity was also measured to obtain some information regarding morphology. Although the rubbery modulus was constant irrespective of a gel-time ratio (Gt) of the epoxide resin to that of BMI for the EM-25 series, it increased with increasing Gt for the EM-50 series. The glass transition temperature (Tg) was nearly constant at Gt < 1 and then decreased with increasing Gt at Gt > 1 for the EM-25 series. On the other hand, Tg was nearly constant irrespective of Gt for the EM-50 series, though the values of Tg at Gt < 1 were somewhat higher than those at Gt > 1. There was an appropriate value of Gt that gave better tensile strength (σb) or toughness evaluated by the area under a stress-strain curve. The Tg and σb were found to be correlated to the turbidity of the SINs. The above mechanical behavior can be explained in terms of the molecular mixing and morphology that vary with a change in the relative polymerization rate of both the resins. © 1993 John Wiley & Sons, Inc.