The biokinetic models used to describe the fate of radionuclides incorporated by humans often lack the support of reliable experimental evidence. Recent investigations conducted in human volunteers using stable isotopes as tracers have shown that some important features of the biokinetics of ingested molybdenum are not taken into account by the model currently adopted by the International Commission on Radiological Protection. Compartmental analysis has been used to develop an improved model which better describes the available data. Major modifications with respect to the International Commission on Radiological Protection model concern the description of the urinary excretion and the values of the transfer parameters describing intestinal absorption and distribution to organs. Separate sets of parameter values for liquid and solid materials are also given.