We report measurements of absolute cross sections for the electron-impact ionization and dissociative ionization of the SiDx (x=1-3) free radicals from threshold to 200 eV using the fast-neutral-beam technique. The deuterated rather than the protonated target species were used in order to allow a better separation of the various product ions from a given parent in our apparatus. A common feature of all three radicals studied in this work is a dominant parent ionization cross section with essentially the same absolute value of roughly 3.7x10(-6) cm(2) at 70 eV. Dissociative ionization processes for all three targets are less significant with a single dissociative process dominating in each case, viz. the removal of a single D atom (SiDx+e(-)-->SiDx-1++D+2e(-)). The cross section for this dominant dissociative ionization channel also had the same maximum value of about 1.2X10(-6) cm(2) for all three targets. A comparison of the experimentally determined total single ionization cross sections with calculated cross sections using a modified additivity rule showed good to satisfactory agreement for all three targets in terms of the absolute values, but reveals some discrepancies in the cross section shapes.