Response surface methodology (RSM) was employed to optimize the drying conditions of horse mackerel dried in a heat pump dehumidifier (HPD). The independent variables were drying air temperature (20-30 degrees C), drying air velocity (1.5-2.5 m/s) and sodium chloride (NaCl) content in the osmotic solution (5.0-15.0%), respectively. The coefficient of determination, R 2 values for specific moisture evaporation ratio (SMER), drying rate (DR), total colour difference (Delta E) and total volatile base nitrogen (TVBN) were greater than 0.89. The combined effects of these variables on SMER, DR, Delta E and TVBN were investigated. The results showed that SMER, Delta E and TVBN varied curvilinearly with increasing of temperature, velocity and NaCl content in the osmotic solution and DR increased linearly. Based on response surface and desirability functions, the optimum conditions for horse mackerel drying by HPD were: drying air temperature 30 degrees C, drying air velocity 1.5 m/s and NaCl content in the osmotic solution 9.9%. (C) 2007 Elsevier Ltd. All rights reserved.