Absorption-desorption-reabsorption hygrothermal cycling was performed in bismaleimide resin. Dynamic mechanical analysis and swelling experiments were conducted to investigate the effect of hygrothermal cycling on the moisture diffusion behavior in the resin. Non-Fickian diffusion behavior was. observed in both the absorption and the reabsorption processes, while the desorption process is found to be Fickian. The molecular network structure of BMI appears to have changed during the hygrothermal cycling. This, in turn, altered the subsequent reabsorption process.