In this study, a heat pump dehumidifier (HPD) was designed and constructed to investigate the drying characteristics of horse mackerel experimentally. Effects of surface load, drying air temperature, sodium chloride (NaCl) content in the osmotic solution, tempering, evaporator bypass air ratio (BAR) and air velocity on the drying characteristics of horse mackerel have been investigated. Experimental results showed that a high surface load of 8 kg/m(2) resulted in lower drying rate (DR), and surface load of 6 kg/m(2) was preferred by considering specific moisture evaporation rate (SMER) and total colour difference (Delta E). DR increased with increase in the drying air temperature. In view of SMER and Delta E, temperature at 20-30 degrees C was optimum air temperature for drying horse mackerel by HPD. However, NaCl content in the osmotic solution and tempering played a minor role during drying horse mackerel by HPD. BAR of 0.6-0.8 obtained the maximal SMER. The optimum air velocity for drying horse mackerel by HPD was 2.0-3.0 m/s. Furthermore, HPD provided an alternative way to process intermediate moisture fillets. (C) 2007 Elsevier Ltd. All rights reserved.