A new modeling approach was introduced for the prediction of pore size distribution development during activated carbon preparation. The mathematical model is based on the modification of a single-pore model for pore growth rate estimation using a population balance and applying a variable structural parameter random-pore model. The model predictions were compared with experimental pore size distributions and conversions at various times for pistachio shell char activation by steam between 800 degrees and 950 degrees C, and the kinetic parameters were estimated.