Characteristics of thin layer microwave drying of apple pomace with and without hot air pre-drying were evaluated in a laboratory scale microwave dryer. The drying experiments were carried out at 150, 300, 450 and 600 W, and the hot air pre-drying experiment was performed at 105 degrees C. Ten commonly used mathematical models were evaluated with the experimental data. The results indicated that the Page model was most adequate in predicting moisture transfer for fresh and pre-dried apple pomace; just a failing rate period was observed in the microwave drying processes; in comparison with the fresh apple pomace, the drying times, or the energy consumptions in the drying of pre-dried apple pomace to remove the same moisture (40%, wb) decreased by 25% on the average at the experimental power levels, and their values of effective diffusivity were higher, which indicates that the pre-treatment with hot air drying can improve the microwave drying rate of apple pomace; four regression equations of drying rate against drying duration or moisture were found to describe very well the drying characteristics for fresh and pre-treated apple pomace respectively; it took nearly 70% of total drying time to remove the latter half of moisture (wb) in the microwave drying with or without pre-drying. The above findings can facilitate the design and operation of microwave drying of apple pomace. (c) 2006 Elsevier Ltd. All rights reserved.