A higher yield of functionalized carbon nanotubes (CNTs) was obtained by treatment of CNTs in HNO3 or H2SO4-K2Cr2O7. The deposition of platinum nanoparticles and nanoclusters on these functionalized multiwalled carbon nanotubes by electroless plating was facilitated by a two-step sensitization-activation pretreatment. The deposition was sensitive to the aging time of the sensitizing solution and the pH of the plating solution. The resulting electrocatalysts were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry. XPS analysis showed that the Pt/CNT electrocatalysts contained 67.3% of Pt(0) and 32.7% of Pt(IV). Test runs on a single stack polymer electrolyte membrane fuel cell showed that these electrocatalysts are very promising for fuel cell applications.