The effect of a progressive increase in the bound strength of moisture in soybeans - a porous hygroscopic solid - during microwave heating is investigated. Thin layers of soybeans were dried with a single mode (TE(10)) microwave cavity operating at 2450 MHz. The absorbed microwave power temperature, and mass loss of the seeds were monitored continuously. Under a constant absorbed microwave power of 0.57 W/g, the temperature of a soybean seed increased rapidly at the initial stages of drying, reached a maximum of about 75 degrees C in approximately 50 min, and decreased gradually during the latter stages of drying to about 58 degrees C in 150 min. To maintain a constant drying temperature the absorbed microwave power had to be progressively increased as required The results indicate that drying increases the average bound strength of the remaining moisture in a hygroscopic material and the energy needed to evaporate a unit mass of moisture increases accordingly. Consequently, progressively higher absorbed microwave powers are needed to maintain the temperature of low moisture solid foods during microwave heating processes that are accompanied by significant moisture loss.