The objective of this study was to determine the mass transfer coefficient during hot-air-drying of celery root under natural convection conditions. The shrinking of the material during drying was taken into account. The inverse problem formulation approach was applied for simultaneous determination of the mass transfer coefficient and effective moisture diffusivity. The results were compared with literature data and the values received based on the Chilton-Colburn analogy. It was found that the inverse problem formulation approach enabled to obtain the most accurate and reliable values of the mass transfer coefficient. It was also found that mass transfer coefficient determination based on the Chilton-Colburn analogy was burdened with gross errors. (c) 2006 Elsevier Ltd. All rights reserved.