Applications of porous silicon are ranging from drug delivery vehicles to micro fuel cells. The size of the pores and their distribution plays critical role in the final properties of the devices manufactured on their base. We performed nondestructive quantitative experimental studies of selected porous silicon samples with gradient porosity. We were able to determine the average size of the pores and its dynamics as a function of the etching time. We also were able to determine the statistical parameters of the pore formation.