The main objective of the study was to determine, based on inverse problem formulation, changes in the heat transfer coefficient during celery root drying under natural convection conditions, and to compare them with those obtained using dimensionless equations. Another aim was to estimate the effects of particle geometry and heat exchange area adopted in the study on the above values. The rate of the process depends first of all on external heat exchange conditions. The solving of an inverse problem enables to obtain much more accurate values of heat transfer coefficient, compared with dimensionless numbers. In this process the heat transfer coefficient determined by inverse problem formulation is practically constant (8.1 +/- 0.4 W m(-2) K-1) and does not depend on moisture content of raw material within the range of its changes from an initial level of 9.200 kg(water) kg(db)(-1) to about 0.3 kg(water) kg(db)(-1). A mathematical model of the heat transfer coefficient, considering a surface water jet, well approximates the target values (R = 0.9509). Simplifications assumed for the simulation models analyzed affect the calculated values of heat transfer coefficient. (c) 2005 Published by Elsevier Ltd.