The deactivation of Ni/SiO2/Al2O3-catalyst pellets in hydro enation of 3-hydroxypropanal (HPA) to 1,3-propanediol (PD) was investigated in a continuously operated spinning basket reactor at temperatures 45-80 degrees C and pressures 2.60-5.15 MPa. The adsorption of by-products and probably also impurities from the liquid feed causes a deactivation of the catalyst by blockage of the mesopores of the catalyst pellets. For the description of the deactivation a reasonable mathematical model was developed. The model parameters were estimated by a method that does not require the activity of the fresh catalyst.