In the present work, a heat pump assisted dryer is designed and constructed to investigate the drying characteristics of various herbs experimentally. R134a is used as a working fluid in the heat pump circuit during the experimental work. Experiments have been conducted on Jew's mallow, spearmint and parsley. The effects of herb size, stem presence, surface load, drying air temperature and air velocity on the drying characteristics of Jew's mallow have been predicted. Experimental results show that a high surface load of 28 kg/m(2) yields the smallest drying rate, while the drying air with temperature of 55 degrees C and velocity of 2.7 m/s achieves the largest drying rate. A maximum dryer productivity of about 5.4 kg/m(2) h is obtained at the air temperature of 55 degrees C, air velocity of 2.7 m/s and dryer surface load of 28 kg/m(2). It was found that small size herbs without stem need low specific energy consumption and low drying time. Comparison of the drying characteristics of different herbs revealed that parsley requires the lowest specific energy consumption (3684 kJ/k(H2O)) followed by spearmint (3982 kJ/kg(H2O)) and Jew's mallow (4029 kJ/kg(H2O)). Finally, dryer productivity has been correlated in terms of surface load, drying air velocity and drying air temperature. (c) 2005 Elsevier Ltd. All rights reserved.