[14C]-Gemcitabine has been prepared as described previously in the original monograph, but starting with [14C]-cytosine. [2H]- and [3H]-gemcitabine have been prepared by iodination of gemcitabine (I) with I2-HIO3 in CCl4-acetic acid to the 5-iodo derivative (II), which is then hydrogenated with deuterium or tritium gas over Pd-CaSO4 in dioxane-D2O, affording the corresponding 5-labeled compound.